Presently have 1,000 miles on a '70 383 basically stock rebuild. Installed the billet rear main seal retainer from Mancini Racing P/N MRE-223 with the belief that was pretty sure way to stop leaks in this area. Yesterday, I was under the car checking the clutch adjustment. There was a light film of oil on the inside surface of the bellhousing and a "partial" drop of oil hanging on the bottom of the flywheel mounting plate.(The clutch is good and dry). A Mopar friend uses a STP-like oil additive to help combat oil leaks. Is this a good idea to try? Any other opinions on how to identify and fix this oil leak problem is greatly appreciated.

Recheck the pan bolt tightness.

Can oil leaking from the oil pan find it's way into the clutch/flywheel dust cover and then up onto flywheel mounting plate, but NOT on the flywheel to clutch surfaces?

Yes it can. Oil can find it's way anywhere. Also take a look at the oil pressure sending unit. If it leaks oil will run down the rear of the engine.

I would suggest no for that & instead open it up/fix it. Not fun but no other way

Try to find out exactly where the oil is coming from The oil leaking from the rear main seal will possibly be on the front of the clutch inspection cover and definetly on the front of the flywheel slung out into the bellhousing I've seen valve cover leaks and oil leaking from the back of the intake valley pan make some guys thing they had a rear main seal leaking, lots of gremlins out there on new builds Hopefuy it is not leaking from the cam plug in the rear of the motor above the crankshaft Good luck Let us know what you find and do to fix it.

Do a search of the forum for rear main seal leaks. Many threads on the topic and ideas how to deal with.