I have a 3.5" Terry Houston Downpipe, it fits real nice and hooks up to the standard exhaust flange. It's the same size at the 4 bolt flange. It does have the intergal wategate, very nice quality piece.
I purchased mine from Cal Hartline (Hartline Performance). Always the best prices and good customer service. He's a very reputable guy that won't steer you wrong for parts and service.