It takes a lot of oil to keep up with the 360 main bearings. I have had shops try to set it up with too much clearance and it would pump the pan dry with a high volume pump. Alway ran a high volume with the 360 mains I ran. I would however try a standard pump with 340 mains.

Are you sure the plug in the drivers side rear of the oil galley is in place. I have seen many left out and had big problems. I also ran a roller cam with the oil galley tubed. Ran Harland SHarps and restricted oil to the heads also. Holley #66 main jet works great on the Passenger side and #72 on the drivers side. The cam I ran had the journal ground to oil the rockers full time. Spun that puppy 8400 in the quarter with no problems.

I think you have clearance extremes or oil control problems that are not getting the oil back to the pan. Wait, truck pans are only 6 quarts. Will a KEVCO pan work they are bigger. Probably 8 quarts.

Another thing if you are running too much ignition timing you may be beating the oil out of the bearings.

Leon