Back to the oil leak.
Here is the spacer in question:

It came in a generic oil cooler gasket kit that I got. I really don't recall whether there was one in the old engine. I suspect there was otherwise I wouldn't have put a new one in the new engine. In retrospect, however, all it would seem this would accomplish is keep the o-ring from sealing against the block, which is exactly what it did. Time will tell whether I really needed it, but so far it's not leaking. I put a paper gasket that came with the new gasket kit in between the block and adapter. And a new o-ring of course.
I replaced the oil pressure switch, too. Nice and dry over there now. I'm a little wary of it now since the original setup didn't start leaking until about 600 miles after I installed the engine. Moving on to the leaky trans cooler line.....