Should I get this?

Even if it's a 318, it might be a 318-3. But -3s can be iffy as well. The newest -3 block I have seen was a re-man out of parts truck with a 1982 casting date. It did indeed have visually thicker cylinder walls but they were thicker on the minor thrust surfaces instead of the major thrust surfaces. My suspicion is that it was originally a marine block. It still looks like it has enough material to safely bore to a 4.04 inch bore, but I would still sonic test beforehand.