Believe it or not, there's a real old school way to tell if a block has thicker cylinders. Knock a freeze plug out that's between two cylinders on the side and see if a standard size pencil will fit between the cylinders. If it won't fit, it's a thick casting. If it does fit, it's not. I know from personal experience this works for early Hemis. My 331 is what's called a "severe service" block, meaning it was cast thick so that it could be bored a lot over its life span. It was a medium duty truck motor, after all. I compared it to another 331 block that was in the same machine shop as mine and the difference was astounding. A pencil wouldn't come close to fitting between the cylinders in my block, but you could fit almost two in the other one. I am certain you can tell the difference in a small block the same way.....whether the pencil is a go or no go, I haven't tried, but if one block is thicker than another, between the cylinders is a sure fire way to tell it.
