When u say "loses oil pressure when it gets hot" do u mean even at high speed? A hot engine is going to have less oil pressure than a cold engine. But at high speed the pressure should be at its highest. How many miles on this motor? Low oil pressure and excessive heat at high speed sounds like a very worn engine to me. Tightning the main bearing caps and/or rod caps are not going to help if u have worn out bearings. They are just torqued to a specific tightness and that's it. I would do a compression test first, on the cylinders just to see what condition the engine is in. Lbs. of pressure that vary between cylinders tells a lot.

'52 3100 "once a Marine, always a Marine!!