You can bank on having to take the engine out and pulling the pan There's just no other way to determine what is going on without doing this.
You can look, stare, and think a oiling problem to death but in the long run you'll be taking the engine out and doing some exploratory surgery, so go ahead while the weather is mild and make plans to do this.
Oil pump pick-up to pan clearance is my first thought. Its possible that its too close>
1/4 MIN - 3/8 INCH MAX
Did the builder go crazy with the silicon? If so the pick itself could be partially stopped up with that junk.
Proper rod and main bearings and clearances, alittle plastigauge goes along way here, like previously mentioned - one wrong size bearing insert can make a big difference and doing a simple plastigauge check on the clearances would have spotted this.
This sounds dumb but is often overlooked, the oil pump cover bolts can sometimes loosen themselves up, never assume they are tight from the manufacturer ( ask me how I know>?) Now I always take them out spray them off with some loctite primer and red
loctite them in place.
Good-luck man>
I hope you get your motor fixed, Charles