3 Bolt Flange, 3.5" OD Collector to 2.5" OD Tailpipe, One Pair

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.