Is something blocking the breathers from doing their job(something in the covers?)? I would think if this thing is building that kind of crankcase pressure it would be blowing vapors/oil out through those breathers. I've seen engines with poor ring seal AND a PCV system push oil/vapor out the fresh air side/breather(more blow-by than the PCV can handle). So I think an engine without a PCV and excess crank pressure would blow that all out the breathers long before it would push past the rear main(or any other seal)

I had a 4.3 GM come in years ago....it whistled out the rear main...lol...didn't really leak much, just whistled . Messed up PCV system. Fixed that and sent it on it's way, never put a seal in it. Just my