Wow! Really crazy you are having so much trouble.
 
One thing comes to mind that I would try is to pull the clutch & flywheel and all again,  pull the dizzy and spin the oil pump with an electric drill for a minute or so to really pressurize the oiling system.  then inspect all your gallery plugs and cam plug for leaks. 

Considering the number of times you have replaced the rear main seal it should have made difference if that was the problem,  It sounds like it's leaking from somewhere more obscure, Pressurize the system and see what happens.
   

 All good advice. Run a re-torque on the head and intake bolts.
 
 Pull the distributor and get a borescope down into the valley, pressure test the cooling system, if you do not see a pressure
 drop or leaks right away let it sit for a while under pressure and re-check.
 
 If the block and heads were shaved during the rebuild you may have an angle misalignment issue with the head to intake
 surfaces, and causing the gaskets to seep.