I always look for high nickle content used in late 60s early 70s blocks. This is probably more important than having 4 bolt mains for most applications.
The good news is that almost all of the 60s and early 70s blocks used for 350s were high nickle content. Look for "010" or "020" cast into the side of the block. "010" represents 10% nickle content, this is most common for 350s (in fact all of the 350s I've ever seen from 69 through 73 were "010" blocks - I've seen pictures of "020" blocks).
The high nickle content yields a stronger cast iron block, similar to the way chromium improves the strength of steel.