I've done six first gens in the past 5 years. Two fenders were late NOS, one was a repro, the rest were assembly line take offs. Late NOS needed a day labor each to correct issues. Take-offs needed the back brace cut to re-arch the fenders to align perfectly with rocker, a very common issue. Repro came with car and brand is unknown. It fit well except for very top of door to fender. Matched the rocker better than any GM product.
Fit is all over the map and varies by manufacturer. Some parts fit very well, and then the company changes sources and the fit and quality is awful. I just went through this with a CHL Dutchman panel that used to be great, now they're trash. Picked up a Dynacorn panel and it was fine.
I'll fix an assembly line take-off first. Tony at Muscle Car Metal presently swears by Dynacorn. But that could change in this industry.