I can tell you this when it comes to fenders, both NOS GM ones and the repros have to be fitted to the car. The worst are GM 67-8 models driver's side rs or non-rs. I am not sure what they got out of place, but almost everyone has needed to be "pie cut" on the inner rear brace in the middle just to get them to fit to the door. The repros for the 67 & 8's are not the best in the world but will fit with some work, less work than the GM ones. The 69's are a great fit, that is if you buy the ones from Goodmark directly, or one of their vendors such as Year One, Camaro Central, Ricks, etc. There are some junk early ones that were produced with the wrong front leading edges and they will not fit the car period. They are usually sold for around $100 - $120 ea and come from either NPD or the Paddock. There are some issues with the 67-68 full rear quarter panel on the passenger side so I highly recommend that you do not remove any of the trunk jamb or the area where it is welded to the tailpanel. Somehow the die got damaged and the stamper for Goodmark decided they would fix it themselves w/o telling anyone stateside. They have it off in the jamb and tailpanel area. All of the others have no fitment issues for any 1st Gen. I know you must use the repro outer wheelhouses on the 69's as there is a slight difference the shape of the quarter panel just behind the rear wheel when compared to an original. It is so slight that it is not easily recognized w/o a template from an original 1/4 panel in that area.
As for the ones stamped from GM tooling....the only ones I know of that have been actually done that way are the 70-72 Chevelle 1/4 panels, and they are as bad as some of the last ones GM produced. The body lines are horrible! Depending upon the type of car I am building, and what the customer wants to spend, I would probably go for the repro stuff except for the 67 & 8 fenders.........JMO..........RatPack................