How rare is a 1970 Grand coupe

With no fender tag or build sheet it doesn't matter what you do with it.

X2. Plus, with no fender tag or build sheet, there is no way to prove it's a grand coupe or what came in it. Even if you restore it perfectly to original, it will never be more than a normal Challenger without proof. That being said, a Challenger is an awesome car no matter what, so I would certainly fix it up and put it on the road.