A 1964 Pontiac block is a good one to use if it is still in good condition. The 1964 389 block has provisions for a 1961-1964 bell housing mounted starter and a 1964-1979 block mounted starter. Avoid the bell housing mounted starter applications because starters, starter drives, flywheels and flex plates for those are extinct.
You are lucky to find a good 1964 Pontiac block because good 1955-1970 Pontiac blocks and cranks have also gone the way of the Do-Do bird or they are in the hands of "hoarders" and antique dealers. .