[Steve Barcak]

you didn't mention application but, the strongest production Pontiac blocks were all cast for the 1958 to early 1960 model year. Pontiac took out a lot of weight in the blocks ( they even advertised it ) for the 1961 model year.
If wanting to use a 3" main, then all 1959 and many 1960 blocks are your best choise. Most can easily go +.125" but, no need to go crazy on bore with a blown deal.
All later parts fit. it has the side mounts. You will need to decide what to do to mount a starter but, that is small potatos for us hot rodders.
20 years ago, my 59 389 block went 6.77 at 200mph on a part pass on a load of 92% nitro, 6-71 and Enderles. these are overlooked great blocks for high horsepower.
Of course, these blocks are no longer plentiful but, they are out there and avail. And no one wants them so, most are pretty cheap.

Best to you.

Pontiac Heaven