I'm going to run the risk, and the certain eventuality, of seeming horrendously biased and self-motivated here. But I'm honestly going to put my "100% objective hat" on, and make some points that I either know to be true, or at least have (in general) experienced over the decades of my involvement in the parts industry..
* The notion of "everyone is basically selling the same stuff" will only serve to sabotage/compromise the finished quality and detail of your restoration. It is an urban myth that just won't go away, and probably never will. The complexity of these parts catalogs, the breadth and depth, of ours and all of our competitors, cannot be overstated. There are SO MANY categories and sections and instances and examples where at least two, and maybe three or four reproductions of a specific part, from huge stuff like sheet metal, to small stuff like lens gaskets or fastener kits, have competing toolings at competing manufacturers, and/or various multiple sources... Manufacturers/suppliers that you've likely never heard the names of, because "brand-marketing" has so-horrendously convoluted and confused this industry and hobby, especially since the racing parts suppliers have dived-into re-selling the product-lines of other branded retailers, who are actually just re-packaging, branding, and re-selling merchandise from a vast array of suppliers. Bottom-line, the differences between restoration parts supplier A and B and C and D are massive, sweeping and scattered all throughout the catalogs and websites. We feed from over 1,000 different manufacturers/suppliers alone. What we have been doing for over 40 years, is REFINING OUR SOURCING, which is the exercise of learning where the "best" of a specific item is being made, and sourcing as-directly as possible to that "best" piece or kit or bag of screws. It has been a decades-long evolution and effort. And I do not state that to brag on NPD, because I would have to imagine that all my competitors have evolved to a degree. But it's that degree that would really separate one company from another to another, as in how much priority they've truly dedicated towards quality-sourcing, and how much success they've achieved in the effort. Man, I'm getting wordy here, I apologize, but this is a heavy subject for me. And if you're quality-minded with your Chevelle, hopefully it's a heavy subject for you.
* This is where I have a hard time walking the fine line. But suffice it to say, one should consider who they're buying from, and whether or not they are as restoration-dedicated as it takes to truly and aggressively practice "best-quality sourcing". If they are simply a massive clearing-house, re-selling other suppliers' ENTIRE product-lines lock, stock and barrel, then that's a big indicator. The answer is "no", they're not. They're just data-dumping entire files into their system, the equivalent of shoveling coal into a furnace to make the fire (revenues) hotter. No thought is placed towards "let's not carry these bezels from ABCD, because WXYZ supplies a far-better bezel".
* That's not to say that there aren't instances where you know precisely what you're buying, the source and manufacturer is known, and it's known that no better-alternative exists, and you can save money. I'm certain those instances exist all over the place. But it's not easy being sure.
* Sheet metal is a good example of how you can get sideways. Most hobbyists only know U.S.-based importer brand-names. But the reality is, that there is a huge network of factories in Taiwan making the stuff, and a large amount of instances where there are two or three competing toolings for a specific part. No single manufacturer is the default "best" on every part. So sourcing for "best available quality" is complicated. We do our best to sort it out. Others just play the brand name game with the U.S. importers. The former approach is far better than the latter.
I did the exercise for about a half-hour, comparing one supplier's website pricing to what their same merchandise was being sold for on another larger supplier's website. And I found pricing to be all over the place, less expensive, more expensive, BOTH way too expensive. A total hodgepodge.
OK, I'll shut up..
I just felt compelled to chime in, and assure you that "it's not all the same stuff". No, not even close. From tie-rods, to fenders, to weatherstripping to lenses and emblems, there are multiple levels of quality out there... Some of it at similar prices such that you'd never know the diamonds from the coal. It's quite vast and complex.
Your dedicated restoration parts specialists are where the rubber hits the road with best-quality sourcing practices, and some have been much better than others. Caveat Emptor.
Thanks for letting me participate.