Installed prochamber on my 1997 GT today
I started this by hosing down all the factory bolts with PB Blaster on Wednesday, then blasting them again each day. I had no trouble getting the h-pipe to catback nuts loose. The O2 sensors all came out easily. Even the manifold nuts weren't bad. The one nut on the passenger side wasn't the nightmare that I'd read about. As long as you have a swivel and some extensions, it isn't bad. I did have to put a cheater pipe on my breaker bar to bust it loose. I think the hardest part was getting the dang rubber hangers off the metal studs of the factory pipe!
Installing the ProChamber would be a piece of cake with a new one. There is much more room getting to the exhaust manifold nuts without the cats in the way. I bought a used ProChamber and it was missing the studs that attach to the catback. The new studs that I bought at O'reilly's just a bit too short. Fortunately they were long enough where they thread into the Mac flange that I was able to unscrew them about a 1/4". I wasted almost an hour doing that.
Ok, so now it's all done and looking good. It tucked up nicely with nothing hanging down any farther than the factory piece. So what does it sound like you ask? Now mind you I still have the factory catback except for a set of Magnaflow mufflers. At idle it isn't much louder than the factory h-pipe. It has a nice deep, throaty rumble though. Revving it up it gets a little louder than before, but still has that great deep tone. I then took it for a short drive. It sounds great in the car! I'm kind of surprised that it isn't louder than it is, but it sounds great.