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back in the day, most auto parts stores sold "test pipes" that were a direct bolt-in for the cat for most any car...haven't seen them in years though. last car I did that to (90 cad) I just cut the ends off the cat and welded a piece of pipe. if you know someone with a small mig they can knock it out in 10 minutes...
Test pipes are legal to market and sale for racing and other off highway applications. Test pipes are listed on both eBay and Amazon among numerous other places.
Just for fun here is a link to eBay's policy on buying and selling test pipes. It does require you to jump through a couple hoops but nothing major.