Okay, here's the latest. The casting on the side of the block is 3769716, which denotes, near as I can tell, a 1958 235. The flywheel takes a 10" clutch and is 13 inches, NOT 13 1/4. It has a 139 tooth ring gear. The rub, apparently, is that the crankshaft is an older 6 volt version (pre '55) based on my current, apparently, incorrect flywheel. The difference between 6 and 12 volt, near as I can tell, is the bolt pattern in the hub and the diameter (13 1/4).

The GM part number that I need is 3705410. The casting number on the flywheel itself is 839754-777. If I can find the right ring gear, I'm in....

Any ideas?