Just pick the reducer size that matches the diameter of the header collector. Bolt it to the collector ring, and weld on the tailpipe. If you have a header without the collector ring, use Hooker
slip–on reducers. They're sized to slip over the collector and weld to the exhaust pipe.
Super Competition Reducer, 3 Bolt Flange, 3.5" OD Collector to 3.5" OD Tailpipe, One Pair
L 8.250 in.
W 4.750 in.
H 4.750 in.
Weight 1.000 lb.