I agree, the build code is to far away to be a left-over motor from 65'. I would think it would make more sense that someone swapped it out in the first few years, for a more performance motor in its time. The 340's were not out yet for that option or it would have been one of those. To answer your build sheet dilemma, build sheets could be found in the back or bottom of the rear seat, above the glove box carboard liner and on top of the fuel tank. I even heard of under the carpet flooring, but I never found one there myself. The other places, yes. Being a different intake and head bolt pattern, I would not think Chrysler would substitute a "A" motor for a "B" motor, unless maybe it was an employee-built car. Crazier things have happened at the plants. Just recently someone had a car painted a color that was not an option. I think it was one of the "Hellcats".