Reasons Are Simple

I think that we've gone over this question before and the reasons for the pressures are very simple and the variations are caused by the velocity of the oil in the main galley. Raising the velocity drops the pressure. With that said number one sees the pressure that the pump can supply up to the relief at high speed but as we go back through the block past the branch for numbers, 2,3 and 4 mains the velocity in the main galley is probably pretty high. This causes the pressure to drop at these places. Get to number 5 and it's higher again because the velocity is lower. I think that's what's happening in the oiling system.

The 351 C and big Ford are not ideal from an oiling standpoint mostly because restricting the oiling to the top of the engine is not as straightforward as in some engine designs. Bushing the right side lifter bores and cutting them back to .020 and restricting the left side(solids or roller only) cures that problem.

The fix for an oil galley without any lifter bores in my opinion would be to have it's bore get progressively smaller as it passes each off shoot to a main. Say start at 9/16, drop to 1/2 inch and then to 7/16 let's just say. I think that would really help to even out the pressures.