1973 was the last year for a 340, that being said it was low compression and cast crank, so not real desireable. Again, that being said, I don't think it is a bad deal. You can change the entire (and I mean entire) front clip from the 71 and it sounds like since the rear end is rusty you can also weld on a 71 rear clip and have pretty much a 71 Duster or Demon looking clone!
The funny thing about science is that if you change one miniscule parameter you change the entire outcome to the way you want it.
