I have been e-mailed about locating high performance/body parts for Camaros and Chevelles. Remember that sometimes you get what you pay for. I have seen 5 different versions of replacement trunk floors, a couple were very light gauge and were without the spare tire bolt carrier.
But there are a few sellers that sell you top quality engine racing parts and reasonable prices. For body parts the usual NPD
http://www.nationalpartsdepot.com/ and OPG
https://www.opgi.com/GateChev4.asp seem to have beter quality and fair prices. OPG is one of the few venders that will offer complete interior/top kits. NPD owners have refused to put large kits together.
For engines, I have found Billy Grahms racing out of Texas to be fair/resonable/ and have good service.
http://www.billygrahamracing.com/
I do not get any kudos from these companies, but perhaps if your looking for fair prices and wanting to save run around time. Good luck.