I'm really surprised no one else has mentioned the bearing thing. When you have good oil pressure on a cold engine that drops alot when hot, that is a sign that the bearings have excessive tolerance. The block and bearings expand as they heat up and that causes the tolerance to expand and thus the oil pressure to drop.
Had this happen to me before and to some degree, on everything ive ever owned. A new oil pump may help, and a high volume pump might even mask the problem for a while but its a bandaid fix.
I'm not saying that this IS your problem, i DO think that it is, but i AM saying that you shouldnt overlook it and that you should check atleast the main and rod bearings while you have the bottom end open. Scratches and visible copper are not good things to be seeing...