Following Ted's suggestion, I pulled one of the heads and used a digital caliper to measure two bores. I guess Ted's experience told him that the caliper measurement would be accurate enough to determine whether the engine is a 292/312. The two cylinders measured 3.796. Upon cleaning the top of one of the pistons, I found "60". I have to conclude that the ECZ-B block is a 292, not a 312. Hopefully, I'll only need to go + .015 on the bores and can use +.025 312 pistons. Sonic-testing for wall thickness and core shift is probably a good idea.