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“I’ve just always been obsessed with the speed and power of cars,” says Steven Engberg of Mount Pleasant, Iowa. “My first car was a ’69 Super Bee with a 440. Back then they were still cheap; I paid like $500 for it. That was quite a handful for a 15-year-old – I got three tickets in my first week of driving!”
A few years later, Engberg happened upon a 1970 Ford Torino Super Cobra Jet. Outfitted with a 429ci big-block V8, a four-speed transmission, and the coveted Drag Pack option, he quickly snatched it up, and the Torino went on to become Engberg’s primary hot-rodding focus for a number of years.
“It was featured in about half a dozen magazines, and I did a lot of racing with it,” he says. “It was a really fun car to have.” But while the Torino certainly satisfied on the performance front, there was another car from Engberg’s past that he couldn’t stop thinking about.
Steven Engberg's wild Boss 600 Mustang is actually his second attempt at this particular car build. The first try was plagued with bizarre and catastrophic issues that forced him to abandon it. But he didn't give up on his dream of a wicked Mustang with Boss power under the hood. “You swear you’re done, but eventually you start to think that maybe the next one won’t be so bad. You stop focusing on the bad parts," he said.
“Back in the day, one of the local street racers had this 428-powered 1969 Mustang,” he recalls. “That car really left an impression on me, so I promised myself that I’d get one once I had the opportunity to do so.”
That promise eventually led him to a 428 SCJ Mustang restoration project. After completing that build and putting in plenty of seat time, Engberg decided it was time to take on another Mustang project with a different approach.
“When I was ready to move on to the next project, I started weighing my options,” he explains. “I was talking it over with some buddies and I was like, ‘I’d love a Boss 429, but I can’t afford an original one, and I don’t really want an original one.’ So we came up with the concept of building a Boss car of our own. Along the way we figured out that a 429 could be punched out to a 557, so it became the Boss 557.”
For his second attempt at building his dream Boss-powered Mustang, Engberg teamed up with a fabricator friend who helped him keep the project on track start to finish. “I immediately knew that I wanted to build another ’69 Mustang, another Boss car,” says Engberg.
While Engberg had a clear vision for the project, the build was riddled with setbacks. “After we got the engine together, we put it on the dyno and it blew up during a pull – the flywheel on the shop’s dyno had a crack in it, and it ended up causing all kinds of carnage.”
Not long after, a mishap with the installation of a bearing in the motor that replaced the first one would take that one out as well. “It was one of those projects where everything that could go wrong, did go wrong. It just got out of control. I only owned it for a few months before selling it.”
Frustrated by the experience, he swore that he was done building cars. But time has a way of healing old wounds, and a few years later, Engberg started to come back around to the idea of building another Mustang. “It was like going through a bad divorce,” he tells us. “You swear you’re done, but eventually you start to think that maybe the next one won’t be so bad. You stop focusing on the bad parts.”
Engberg gave his sinister Boss Mustang the nickname "Elvira" – the mean younger sister of Eleanor.
One day, a fabricator that Engberg knew through a business relationship asked him if he’d ever thought about building another car. Engberg told him about the problems he’d encountered with the last project, and together the two of them devised a plan to work on the project side by side in the evening at the fabricator’s shop.
The proposal addressed two of Engberg’s primary concerns – by being directly involved with every step of the project there’d be no surprises with the build along the way, and the plan would also prevent costs from spiraling out of control.
“I immediately knew that I wanted to build another ’69 Mustang, another Boss car,” he says. “But this time around, I wanted to take it as far as I could possibly go – I wanted to build it to a level that would ensure I’d never want another car.” Dubbed the Elvira Mustang – Eleanor’s younger and meaner sister, says Engberg – the project took on more of a restomod vibe than the builds that he had put together in the past.
“I wanted the car to be more of an all-arounder,” he explains. “I wanted it to handle well, have great drivability, perform at the drag strip, and do well at shows. And we wanted to make it timeless – something I could take to a car show ten years later and it would still feel relevant.”
This time around, Engberg wanted to build a car that could do everything well, in the hopes of quelling his desire to replace it quickly after finishing it. "We wanted to make it timeless – something I could take to a car show ten years later and it would still feel relevant," he said.
Originally a ’69 Mach 1 that was purchased as a roller back in 2004, the team set to work creating something that would take the Boss 557 concept to the next level. The chassis was a keen area of focus for the team.
To take the Mustang’s corner-carving capability to the next level, they chose a TCI Mustang II front suspension as well as a subframe from Detroit Speed, while the rear was outfitted with a Detroit Speed four-link. A four-point cage was also added for safety as well as additional structural rigidity, while the Mustang’s stopping power was vastly improved by a Wilwood big brake kit.
Motivation is provided by a fuel injected Jon Kaase 385-series Ford big-block V8 with Boss heads. The motor displaces a healthy 598 cubic inches and dishes out roughly 800 horsepower by Engberg’s estimate.
The power is channeled to the rear wheels by way of a Tremec six-speed manual gearbox and a Ford 9-inch rear end with 4.11 gears, a combination that undoubtedly results in a very entertaining street machine. But as brutal as the powertrain is, it’s the aesthetics that really set this car apart from the crowd.
Elvira is powered by a fuel injected Jon Kaase 385-series Ford big-block V8 with Boss heads. The motor displaces a healthy 598 cubic inches and dishes out roughly 800 horsepower by Engberg’s estimate.
“People look at the car and they know it’s different, but they can’t really put their finger on why,” he says. “I wanted it to still look like a ’69 Mustang, but cleaned up.”
To achieve its sleek look, the team got rid of the seams on the fender extensions, removed various pieces of factory brightwork and the side markers, and ditched the factory door handles for flush pieces from Kindig-It Design. Out back, the stock rear bumper has been replaced with a carbon fiber piece that’s more closely fitted to the body, while custom taillights help give the rear end its distinctive look.
Engberg wanted the car to look essentially stock, but better. To do so, he gave the car plenty of subtle body mods, including removing the seams on the front fender extensions, eliminating some of the factory brightwork, and replacing the factory door handles with flush units.
“I also wanted the engine bay to be like a jewelry box when you open the hood – like all you would see is this gleaming gem inside,” notes Engberg. “So we flattened out the firewall, removed the factory-style shock towers, and fabricated a custom cover that sits between the grille and the radiator support. We also reversed the reservoirs for the brake and clutch and ran all the wiring through square tubing to clean it up even more. I wanted to make sure that your eyes are drawn to the engine and nothing else.”
To make sure the massive Boss powerplant got everyone's full attention when opening the hood, Engberg did a number of mods to visually clean up the engine compartment, including smoothing the firewall, rearranging the clutch and brake reservoirs, and running the wiring through tubing. "I wanted to make sure that your eyes are drawn to the engine and nothing else," he says.
Since the build was completed in 2015, Engberg has taken the Mustang to a variety of different events, earning a Platinum award at the Muscle Car & Corvette Nationals and a Best in Show at the 2022 National Mustang Racers Association Nationals along the way.
His original goal of building a car that wouldn’t leave him wanting for more seems to have been realized – eight years later, the focus is on keeping the car dialed in rather than making any major changes. But he’s not ruling out the possibility of building something even gnarlier down the line.
“We’ve been kicking around some ideas. I was thinking of maybe doing a red ’69 Mustang and seeing if we could take an aluminum block to 666 cubic inches – we’d call it the Boss 666 and name it Damien,” he quips. “That’s still at the theoretical stage at this point, though. The bar has been raised so high with builds like these in recent years. For now, I’m just going to keep having fun with this one.”