340 or 360 Thick walled block.

Yeah, I should have clarified a bit better, and I agree 100% on the early hemis, I have a 331 here that is super thick. The production blocks are a crap shoot, my experience is that you cant simply say "early 360 blocks are thicker" some are, some are not. I have a 90 roller 360 here that's as thick as any "early" 360 I have checked.
Yup. Sonic testing is a must. In fact, I'd go so far as to say it should be the first thing done on every single block after coming out of the cleaning vat.