Welcome to CHR!
You start with saying, “…a newly rebuilt SBC 383.” When I read that I assume that you have a 350 bored .030 over and a 3.750-inch crank from a 400 small-block. A bit later tell us it’s been bored .010 – does this mean that the engine failed and now it has .040 oversized pistons? You also indicate that the oil pump was not changed (during the re-bore?) but you have a high capacity pan. I’m a bit confused as to the sequence of events but there are several indications of a mis-match between the pump/pickup and pan. The other possibility would be if you started with somewhat uncommon 4.005 bore and a 3.800-inch stroke (Chevy high performance engine – very expensive!). In either case, I suspect an improper oil level at speed and you’re experiencing either starvation to the pump or cavitation causing the oil to flow up the sides of the block and away from the pickup. Here’s where I would start:

Replace the following components:
Pump - Miloden 18756 high volume/standard pressure.
Pickup - Miloden 18314
Pan - Milodon 30901 seven quart pan that fits stroker motors – assuming passenger side dipstick.
Gasket – Miloden 41000 one piece
Windage Tray – Milodon 32102

Optionally:
Milodon 23050 oil pump drive shaft
Milodon 81300 is an oil pan stud bolt kit – worth the $20 for ease of assembly (in my opinion).

Let us know what you decide –

Regards,
Glenn