51 Jimmy,
Chevrolet didn't start using frame numbers until 1955...so your truck will not have them.
If the ID plate is missing and the engine has been swapped there is no way to confirm what you have really. Features that could help ID it by year are all bolt on items (doors, door handles, cabs etc) so it's going to be hard to convince someone of exactly what you have...who knows what changes have been done over the last 65+ years???
Good luck,
Mike B
