the big problem as i see it is the handling. these cars handle like shopping carts right off the show room floor. age then 40+ years and drop any motor in there and its going to be a sloppy pig. everyone of these cars needs a front end re-build and stiffer tortion bars no matter what you intend to power it with. then i hear the crying about brakes. anything more then flintstone power needs a disc brake upgrade. hell even your 300k mile slant six running on 2 cylinders can gather enough steam to make a panic stop with factory drums a pants soiling experience.
