1995 Firebird Exhaust
For headers, you probably won't find a collector bigger than 3 1/2".
Flowmaster sells a 4 inch merge collector. You can get a custom y-pipe that is 3 1/2" at the header collector and use the flowmaster merge collector for a 4" out. I don't know of anybody that sells an already made y-pipe like that.
P.S. I want to know your setup that requires exhaust that large and don't say you have a stock engine.

