What reproduction parts fit the best on the 1st gens? I see so many out there but don't know what to buy. My car is in desperate need of some rear quarters and trunk.

I think you'll get a general concensus that the aftermarket products do not fit as well as the original GM parts. While aftermarket body panels are much cheaper, be prepared to do a lot of "tweaking" to get them to line up and fit properly.

The rear quarters, deck lid and tail pan that I got from Goodmark fit quite well. Nothing will be perfect. You will see fenders for $750+ that they claim are from original GM tooling but they still need work and end up costing you a lot more in the long run than the Goodmark.