Its inspection time for me; which means its time to bite the bullet on a catalytic converter. I ruined the current one driving around on a misfire for, oh, say a few thousand miles. Any chance of doing this myself -- i.e. bolt on, or clamp in; or must it involve welding? I'm getting quotes of over $400 even for a universal. I'm riding a rejection sticker and have to go to a (NJ) state ERF (emissions repair facility), so I don't know if that's why. And yeah, I know I probably should've fixed it first...

Weld it in. Most muffler shops will install a universal TWC (Three Way Catalyst) for well under $200.