I agree, the G28's nowdays are terrible, especially with all the splashes they've gone through. My body was the 3rd body ever ran from the original moulds, so it was in pretty good shape from the start. When the moulds reached the end of their lifespan was when Triangle-G sold them to CarkitInc, and by then they were warped and difficult to allign. Joe at I.E. came up with his version of the G28, but did modifications to the body that actually made it less accurate and harder to assemble, all in the name of making a more affordable "budget" body (i'll never understand how making the body less accurate and not providing any mounting flanges for the parts make it easier or any more affordable though).
Everybody has always wanted a 100% accurate Murcie body, but until recently the only one was the Extreme kit, but with shipping costs plus the price of the body, it was unaffordable for most builders.
If I ever did another Murcie, I would definately go with the A/D (now Jayso) body, but I would NOT build on a Fiero though. Theres nothing wrong with building on a Fiero, I believe its a great and reliable platform. Building a A/D body on a Fiero would be a very long and expensive build though, since the cockpit of the Fiero is totally wrong to fit the A/D Murcie body. By the time you change the Fiero to fit the body, it would have been faster and cheaper to build on a tube chassis. Not only would the rear firewall need to be moved up on the Fiero, but the front firewall as well. That would mean relocating everything from the steering column and rack, to the master cylinder and A/C (not to mention all the restructuring and bracing of the new firewall).
The G28 was a popular kit because it was close enough to the actual car (length, width, wheelbase, height), and could be made darn near perfect with just some minor body changes. It was made to bolt right on to a stretched Fiero, which have always been redily available and affordable.
I'm anxious to see how your car comes out, and I hope you post a build diary to show your progress