i'd take it to the shop and have them fab up an ass-end 2.5" pipe to run to the muffler (it is a 2.5" inlet muffler, right?), weld in a cat and o2 bung, and call it a day for now. Have them look at all the flanges and gaskets to make sure there are no major leaks too. That should get you through the sniffer test, and if you feel like upgrading, you can do that later and save some money for now.