Well, since Mike brought up aftermarket H and X pipes, it depends on the shop that you go to. If I remember California law, you are not allowed to remove functional smog equipment. If you bought your car without the right emissions equipment, you are then allowed to use aftermarket parts. So you could argue that replacing your broken OEM h-pipe with an aftermarket one is OK in that case. But, unless the smog tech owns a Mustang there's no way they will know if your h/x is factory or not.
Technically you're supposed to have 4 cats, but I have yet to run into a shop that counts them; they just check to see if you have cats or not. I had a GT (which I've since sold) that passed several times with a Magnaflow H pipe, which only has two cats. While catalytic converter tech has improved quite a bit since our cars were made, you do take a risk that the two newer cats may not be enough to pass the tailpipe sniffer test.
As Mike said, a cheap fix would be to try for an OEM h-pipe on Craigslist.