Low peak oil pressure

My suggestion would be to use an inspection camera in the valley area while priming to see if that lobe lift/lifter pairing(pre-magnum block?….actually, it has to be a magnum block to use those lifters…….right?) is causing excessive leakage around the lifters.
(More lift is created by lowering the base circle, so the lifters would be lower in the bores when the valves are closed, which could creat a path for oil flow from the oil band out the bottom of the lifter bore)

Probably need a helper to turn the motor over while using the camera.