There should be a stamping up front, right under the pass head, just over the water-pump port. If it's a replacement engine, it may not show anything there, and casting 'Hecho Mexico' anywhere were the Goodwrench replacement motors. Those that I was familiar with were the '010' casting, (last 3-digits)
They're were 2/4bolts. The 2bolt ones had more meat in the main webs for splayed caps that are said to be stronger than the 4bolt ones..
For the next block you look for, Set the block on end (waterpump side down) and look right down at the cam.. Is the hole centered? If it's not keep looking. CORE SHIFT is very prevalent. I sold a really nice block to a guy that said he was going to build a motor for his daughters car, then traded for a rebuilt Jasper one.. Man, I could have given him a junk one if I knew he wasn't going to use it. There was just something about that block, everything just looked good on it..
Anyhow, Keep away from crank casting #'s that are the same from 305's and 350's.. The balance WILL BE OFF!! (ie. '442' casting)
I've got a few older books for casting #'s.
Sometimes the castings marks are just barely readable.
I can take a look fer ya.
E