bear with me John, I'm alittle slow. You say that heat melted a valve cover gasket, which turned to a molten mass and blocked the rear (about 3/4"X1/2" passage? Which starved the main bearings of oil?
Just curious, cuz I cant for the life of me imagine how that could happen. The amount of oil going to a head is miniscule compared to anywhere else. The little bit the pushrods transfer is to lube/cool the RR. Oil should be draining thru the numerous holes in top of the block. And as far as the oil in the head draining back, there are 4 other passages (much larger passages) that oil could get back from the head.
I am not doubting you or your mechanic, but even if you filled the head full of oil, that cant be more than 2qts. You have a 7qt oil pan, no? Almost every hardcore racer runs they're car 1qt low (yes they do, so quit shaken your head

) even then you'd only be 1qt low. I've had a valve cover gasket leak, and you know it from the smoke rolling out from under the hood.
Have you already took it apart and inspected the main bearings? This could be something super simple imitating something serious. (I dont know, but if you dont look, how do you know).
All and all, I would say your oil pressure was high from the giddy up, 80psi seems way to much for a small block chevy.
If you've made up your mind, thats cool. But just me playing the devils advocate, I'd say there is no way hell that starved the engine. Now did some of it work its way down into the lower oil gallery, possible????
Rob