From page 14 of David Vizzard's book, How To Rebuild Your Smallblock Chevy. Just so you know Steve this is only for smallblock displacement up to 400ci
"The best way to identify a block casting when hunting for a specific one is by the block casting number. This is normally located in two areas; on the flywheel flange at the back of the block on the right side when viewed from the front of the engine, or in the area directly beneath the cam sprocket. Of course that's not an easy one to read when the block is together so the one on the back of the block is the one to check. However, there are some ways of roughly identifying what you are looking for without having to store a whole batch of engine numbers in your mind. If the engine is out of the vehicle, you can establish whether or not the engine is pre-68 or 68' and later engine by looking at the crankshaft flange where the flywheel or torque converter plate bolts on. If it is a small counterweighted type flange, it's a 68" or later engine. If it's a plain round diameter, it's a pre-68 engine. Of course this won't actually tell you the displacement, but it will establish whether it has small or big bearings. As far as 400 cid blocks are concerned, you can tell a 4 bolt block from a 2 bolter by the number of freeze plugs on the side of the engine. Three freeze plugs along the side indicate the block is a 400 with four bolt main caps. Two freeze plugs indicate a two bolt unit. To pinpoint blocks any further than this, you'll need to check the block casting numbers."
