Figured I would post an update on thread since I left it hanging.
I sent the original pan back to ACE, and after a lot of back and forth, he sent me another one.
This time I also built a fresh 6.0l to replace the 5.7l that lost oil pressure. All the gaskets are new, and I replaced the rear cover. When I put the new pan on, I noticed it also didn't lay flat until I bolted it down. I meticulously torqued the pan in sequence, and installed the engine. It leaks in the same place as the last one. Dribbles oil all over the passenger side header, and some over on the oil filter.
I pulled everything apart and took the pan off (again), put RTV on both sides of the gasket, installed it finger tight and waited 12 hours. Torqued it in sequence again and put it all back together. Still leaks in the same place. I've been running it like that since there's nothing else wrong with it, but it's making a huge mess.
Yesterday I decided to try changing the crank position sensor, since that's one of the few parts that was in common with the old engine. No change. It also appears to be dry by the dipstick tube and the oil level sensor plug.
I'm open to suggestions to fix this (I'm super tired of it...). Some time when I have nothing better to do, maybe I'll bolt a stock pan to it and run it for a while to see if it leaks. That seems like it would confirm that it's the pan, or that it's something else. Of course that would also cost me 12 quarts of oil (6 to fill the stock pan, then 6 more to put it back together), and I couldn't drive it like that since the steering would be taken apart.
Since this is a completely different engine, with a leak in the same place, it indicates to me that there's a serious process problem with ACE. Two pans, two engines, same leak. I could be wrong, but I can't think of anything else that could be leaking. The rear cover is brand new and aligned perfectly with the block surface. It's not coming from the oil pressure sender, the cam sensor, or the crank sensor. I have put together 3 other LS engines and not had any oil leaks. I've owned two other stock ones and had no leaks.