If you have 50 psi at the pump and not getting oil to the rockers then your lifters are not receiving sufficient oil through the oil gallery in the block. It could be a heavy sludge builup because of the unusual oil flow in the 4.3 engine.I have an idea which you might try. Take 1 quart of automativ transmission fluid and pour it into the engine oil.Transmission fluid has a very high detergent factor and will clean most internal parts inside an engine in a couple of hundred miles.Trust me...it won't hurt anything.I do this to any engine i am about to rebuild in the coming weeks,and i get great results.