You can cut the existing pipe and put the cat in with two muffler clamps, or as you said, buy the cat you want, and have a muffler shop weld it in for you. I'm not sure that by law, they can weld in anything other than a stock 88 Porsche catyletic converter, you may want to call around first.

I've had good results from the Magnaflow universal cats. They have CARB EO number so are perfectly legal replacement for CA.