Is the "pad" on the passenger side of the engine the one that's right above the oil pan? That's the pad that should have at least the last part of your VIN on it....if it's not there, it's unlikely that you have the original engine. If there is a large blank space on that pad, then you might have a "warranty" replacement block...not uncommon. Your trans should also have a pad on the passenger side, with the last part of your VIN also.
Finding a build sheet for another car in yours is quite common. I had 4 build sheets in my AAR, 2 correct ones and two for other cars. My 68 Roadrunner had 3 build sheets....two correct, which were behind the rear seat and under the front seat, and an incorrect one, which was under the rear seat, as yours was.
Good luck with your new toy!