Thanks everyone! It is indeed located on the top rear of the block, just behind, under, and to the left of the distributor. Very well hidden. Oil pressure readings have returned to normal, and if this OPSU gets anywhere near the 198,563 miles the original did, I'll be very happy indeed!
FYI......The Haynes manual describes the location as "above the oil filter", so I was under the truck looking for it. I suppose the manual was "technically" correct but that was a poor description.