I think you got the break in question because from what you describe it appears to be acting like the crank case is getting pressurized somewhat when you get on it hard.

Years ago I had one do that whenever I'd get on it hard. It would cause oil to seep through the windage tray gasket setup and sometimes would even blow a little through the valve covers. I was concerned I hadn't gotten the rings to seat properly and was starting to think I needed to start over.

See the picture of your engine could I suggest changing out the driver's side valve cover setup to a breather cap rather than the twist to close filler cap and see if that helps any. Also make sure you've a good strong vacuum signal where the PCV hose is hooked to the engine. You may even need to add a second breather cap.

Switching that stuff around stopped the problem for me and the engine ran great for years.