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, Slip–On, 4.5" OD Collector to 4" OD Tailpipe, One Pair
Sized To Slip Over The Collector And Weld To The Exhaust Pipe
L 8.250 in.
W 4.750 in.
H 4.750 in.
Weight 1.000 lb.