It all seems so simple and straight forward.
You find a super valuable piece (exa. COPO, L88, etc.), but, it needs some or most of its original body replaced. It is documented, has the original drivetrain and a known history. If you considered parting it out because it would need a re-body, you should seriously consider having your head examined. Thing is, you wouldn't want to use some aftermarket panels, you would scour the planet for either NOS or excellent used original pieces. If you had to cut out some super secret # and weld it in, sure, do it. Make it indistinguishable from original. During your ownership, it would be at your discretion to let anybody know (or not). But when it comes time to sell, there is only ONE correct thing to do, disclose it.
As for buying just a trim tag and documentation, well, go for it. Do it right, but, when it comes time to sell, same rule applies, you gotta be honest.
That's the way I see it. My L88 had (has) a hit motor in it. So what, the original only lasted a short time. The buyer was told, in no uncertain terms, it is a re-stamp. I still got a boatload of big green daddyoes for it.
Regards,
Mark Donnally