The poly would make it a resto mod. Polys are Polys not hemis, but, semi-hemis.
AFAIK; the blocks,cranks,and rods,are all the same as regular LAs. The specialty parts are the heads,cam,and pistons, plus the bolt-ons.
But honestly, I can't see a Poly in a 64 narrow engine bay. How are you thinking of handling the exhaust,and the compression issues?
Magnums fit pretty easy
Good luck in your search