It's getting me stumped. It doesn't sound like you have any plugs  missing. The only thin I can think of at this point is that the main bearings have too much clearance and are letting too much oil by. I know they are new, but even new parts are occasionally built wrong.

Have you checked the relief setting on the oil pump? How about the one in the block?  Did you put any sealant on the front main seal where it mounts on the crankshaft? Did you put in a new oil pressure gauge?

Just trying to brainstorm here I guess. Let me know.