the "downpipe" your speaking of is actually the b-pipe. from the cat and back..those measurements are in OD. not ID. header is measured in OD as well. the cat however, is measured in ID. to answer your questions..a 2.0" collector usdm header of your choice(DC sports?) would be a good idea. also, i would highly recommend getting full 2.25" OD tubing to go with your 2.25" OD axle back. once you have that...get a 2.0" ID cat from car-sound. with that setup...you will be creating an anti-reversion setup with the stepping up in ID. hope that helps. try reading the articles on exhaust basics and such. they should help