Charlie , I`d go w the 73 455 , but would not worry abut the a/c, just get a Vintage air, that way you`ll always be able to get to the 7 and 8 spark plugs....talk to Drew about the 455 , he built his pretty well, also Lee Atkinsons (which is an 11 sec car)
with the right heads and cam you`ll be fine...just make sure the blocks arent cracked...
plus Q or Banshee may give you a lot better and more detailed advice...