The 232 you had is a seven main bearing engine, and has a good reputation as far as durability goes. If the crank was ground off center on the journals that were damaged, that could account for it. Possibly an imbalance of some sort could have set up harmonics that might have caused some kind of wear problems- but that is admittedly a long shot. Otherwise, given the use and maintenance it was given, I cannot come up w/any other reasonable explanation.
There are some real sharp Rambler/AMC guys here, hopefully one or more will have a comment that will be of some use to you.
Was the crank salvageable or did you just replace it outright?