Hi,
It looks as if you've gotten some good suggestions for things to look for.
Here are a couple other things you might want to check:
Look at the surface the front crankcase cover seal rides upon, on the extension hub. If it's been roughed up, due to leaks or dirt getting in there in the past, a new seal will quickly get tattered and start to leak. The only soution is to smooth off the hub area the seal rides upon. If it's heavily gouged, this might mean adding some material to bring it back into shape or replacing the entire extension hub. A new seal will then ride upon a smooth surface and won't get damaged.
If the oil pan or sump bolts are ever overtightened, the area around each bolt hole can be deformed. Most often it will dome upward toward the block and make a good seal next to impossible.
There is a "leak finder" product on the market, which is a UV dye that's added to the oil and makes it easier to trace back the source of a leak. Of course, a freshly rebuilt engine might be clean enough that you can trace it back visually.
Good luck!
Alan