I am not familiar with your particular motor but many have an exhaust crossover passage. If that casting gets a pinhole in it, it can pressurize your sump and give the impression of blowby even if the rings are in perfect shape.
The crossover is left over from automotive design from which the marine motors originated, and the evolution of the GM based Q series of small block marine motors eliminated this crossover. Virtually all of the Edelbrock aluminum intakes made for the Ford FE series, for example, have the crossover eliminated, and many have blocked it off with a piece of stainless steel shim stock because it really does not do much if anything for a marine motor. Yes it does allow for motors to warm up faster, but it also cooks the carb and adds a lot of unnecessary heat in the motor box. If you have the crossover I would certainly check it, and also if you don't then check the intake (and head) gaskets because they can potentially leak the same pressure into the sump, depending on the style of intake used.
1956 17' CC Sportsman, 300-hp
1957 17' CC Sportsman, 95-hp
1966 20' CC fiberglass Sea Skiff, 210-hp+
1973 23' CC Lancer inboard project, 427 power.
1966 38' CC Commander Express, 427/300-hp(2)
So many boats.........so little time.....but what a way to go!!