Alright, so now the case is split.
I dropped the bare case in the parts washer, then went to start pulling the old rings off the pistons. They're all pretty loose, except for the piston that was seized. I started with that one, then immediately found this:
Yeah, this piston is junk. Looks like someone dropped it, then installed it anyway. The side is cracked, and the skirt is all bent up. AMPCO is not OEM, as far as I know, so this engine has actually been rebuilt in the past at some point, likely when the car was only a couple years old. This is very discouraging, since I can't just go and buy a set of pistons for this thing. I have many ebay search alerts on, and have searched many part numbers in foreign and domestic sights, but I have yet to come across even a single piston for sale. I know they're out there, if anyone has any leads, please let me know. In the mean time, I've been cleaning everything over and over.
The cases turned out nice, they still need another round or two of cleaning before they're ready.
This is more or less where I'm at now. I have obtained crank and rod bearings, so I will be pulling the crank apart to clean and rebuild it. I will be able to get the short block together, and I will be cleaning and re-doing the heads as well. After that, I'm stuck until I can solve my piston problem.