Probably going to need an adapter. PaceSetter downpipes come with one, which takes the 3" pipe down to 2.5" allowing you to kinda half-ass bolt it to stock cat. Why'd you get a short downpipe if you're gonna put a test pipe on anyways?

For all emissions purposes, get your sticker with stock exhaust (if u have emissions to pass) And then get any good quality downpipe (Apex'i N1 and GT, RRE, and so many more) which include a 12" extended length of 3" wide pipe that REPLACES your cat entirely.

Example, an Apex'i N1 Downpipe and Cat-back exhaust has your exhuast flowing 2.5 - 3.0" from after your o2 sensor housing back. Sold for about $700 shipped for both.

-Nick

'95 Copper GS-T "Serena" Stage 3 CM Clutch w/alumin flywheel, Injen Intake, Apex'i I/C pipes + Twin Chamber BOV, Defi D-Series Boost + A/F gauges...pushing 17psi!