Well, so much for the no leak report from Friday. Sunday morning I took the car into town for breakfast, runs great, sounds good, and has plenty of power. (Sorry, no smokey burnout videos!)
Came back and parked it in the shop and did a few other things for an hour or so. When I glanced under the car, there was a 4" puddle under the rear of the engine, and some drips under the trans and diff.

Crawled under it and sure enugh, the oil is coming from the rear of the engine, dripping off the bellhousing. That kind of ruined my day.
Was busy yesterday, but today, I went down, moved it to the back of the shop and jacked up the front and put it on jack stands. Once under it, the oil is obviously coming out around the crank seal area, but when I pulled the bellhousing cover, the crank flange and front face of the flywheel are dry. I checked the bolts on the oil pan, and all are tight.
So, I did some searching on the problem, and I think I have it figured out. When I assembled the engine, and installed the rear main cap, I failed to apply sealant to the area at the rear of the cap. I did apply a small dab of sealant to the ends of the crank seal, but failed to apply any to the area between the seal and the outer edges of the cap. On Chevy small blocks, the oil from the pump is routed into the rear main cap and then up into main oil galley of the block. There is no seal between the cap and the block, other than the metal to metal surface between the two. The chance of that surface providing a perfect seal for the oil, is literally zero.
So now I am looking at pulling the engine to fix my screwup. Tomorrow I'm going to examine the area closely to see if there is any way to possibly clean the area with brake or carb cleaner, and somehow apply a coat of sealant to the parting surface to stop the leak, but I'm not hopefiul of that. If not, I'll get started pulling the engine and transmission.
I think I still have the box the gasket set came in when I assembled the engine, and I'm going to look in it to see if there are any instructions on applying sealant to that area when installing the rear main seal.
In my defense, it had been quite a few years since I had rebuilt a small block, so it was just a small, but important detail I missed.
On the bright side, I haven't seen any other leaks anywhere on the engine.
Pictures will document my task of shame.