as far as the gauge is concerned you should bleed the air out of the line where it goes into the gauge.
There are several oil galley plugs that need to be in place. there's one they mentioned in the rear of the left lifter gallery. You should see it just above and in front of the intermediate shaft gear. I can't remember for sure but there is probably one on the rear of the Rt gallery, too. There's another one below(block inverted) the rear main cap. When this plug is in place it directs the oil from the pump and to the oil filter. If it is left out, you will get only partial oil filtering. I doubt that that is your problem, because it wouldn't have much to do with oil pressure, just filtering (that's important, too). There's another one, I think, in the oil filter boss of the block.
Did you use Teflon tape on any of the fittings? You have to be careful with Teflon tape. You have to wrap in the correct direction, and hold it away from the first thread or two. If a Teflon booger hangs out it can block an oil passage.
Did you prime the oil pump with oil when you installed it? I pack the oil pump rotors with Vaseline. It stays in place until you're ready to finally prime the engine after it is installed.
There is a remote possibility the oil pump relief valve could be stuck partially open.
