While any block will certainly do, neither of those guys really answered your question. Some blocks had thicker main webs than others, so here are the casting codes for the ones with thick main webs and a little description of the running changes:
1) C9 has bosses cast for 4 bolt mains, 9.48 deck height
2) D1 has bosses cast for 4 bolt mains, 9.503 deck height
3) E3 block used in HO trucks, 9.5 dh
4) E4 block has one piece rear main seal, replaced the E3 block in HO trucks, 9.5 dh
5) F4 truck block has one piece rear seal and roller lifter valley, 9.5 dh
The E9 block is also pretty stout, so if you run across it first, don't walk away.
How much effort you're willing to put into finding one of the better castings is up to you, but you shouldn't pay extra because good ones are everywhere. There are in fact only 2 castings I would avoid, the D2 and D4. That's mostly because there are so many better ones out there you don't need to settle. However, if someone gave one to you free, that's a good deal. Also, any block can be converted to a one piece rear seal.
my preferences are the C9, D0, D1 and F4 blocks. the C9 and D0 blocks are the 9.48 and the D1 and F4 blocks are 9.5 deck height. i'm currently looking for an F4 roller block to build my long rod roller motor with, this engine will be going in a 69 cougar.