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.
REDUCER FLANGED - Part number: 11040HKR - for American Cars - delivery to the UK and the EU
Collector Diameter (in.)
Package dimensions Width: 4.75 inch Height: 4.75 inch Depth: 8.25 inch Weight: 2.00 lb