They were built from 1970-76. The highest horsepower one is 1970, 400hp 550 lb/ft
but that one needs leaded high octane gas. I would go with like a 1972 motor 245hp i think and like 365lb/ft. that one doesnt need leaded gas, but it may need premium.
None of the 500/472s require leaded gas. The early 10:1 motors require premium, later 8.5:1 motors like regular. In reality, the 8.5:1 motors make 300hp and around 490ft.lbs., while 10:1 motors make 15-20 more hp and torque. However the stock exhaust system and bead-catalyst convertors really robbed HP, so if you don't have to pass emissions definately run at least a good dual exhaust system. But yeah, any year motor will be fine.