The studs are splined, and if you hit it hard enough they should come out. Be carefull if you are working on the cat, as the vibrations might damage the catalyst matrix inside. I too have used lots of heat and PB Blaster, and eventually got the studs out. One required drilling.
Replacement studs can be had at a Ford Dealer for the cat itself, but I dont know about the rest of the system. The studs are not cheap: I seem to remember $5-10 each, and you need 2. Better than a new cat though...
