How rare is a 1970 Grand coupe

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.

Does it have a VIN plate? If so, and the VIN has "BP" as the first 2 digits, then its a Gran Coupe. ("BH" is a Barracuda, "BS" is a 'Cuda). The 5th digit will give you the engine size. After that, everything will be a guess.