I would say run the Vin to see what it actually is as far as frame. The biggest thing about the price here that i see, is how good was all the swapping done. Not too sure about when they switched over the rear tail lights but i know my 72 has the round rears. That would tell me that they had to change out the rear crossmember for some reason( rust ).I would not spend that much for it. Too much work has to be done to it to get it right. ( body wise at least ) I would offer 2 grand.
HTH
HTH