The 318 that was in my 87 Diplomat had a similar issue. Minimal bore wear, no ridge and broken top rings.

In this case the exhaust flapper was stuck shut forcing the exhaust from that side to use the crossover in the intake as it's path out. Engine ran hot, but not boiling over hot. I assume, I know, that the added heat caused the rings to expand enough to break. New set of moly rings and fixing the flap was the solution.