IMHO, you should check the head gaskets, this is the most probable source of the leak. As far as I know, the other common way to get fluid transfer between the cooling and lubrication systems is via a crack in the block or one of the cylinder heads. The only other possibility I can think of is if you are using some sort of heat exchanger that uses an automatic transmission-type cooler imbedded in the main radiator to cool the engine
oil. That's pretty uncommon, but if you are, you might check that heat exchanger for a rupture.
When the builder rebuilt your engine, did they check the mating surface between the block and heads to ensure they were parallel? If the mating surfaces don't match properly, the gasket may not have been able to compress properly and this will certainly make it susceptible to a relatively quick failure.
Do you have any idea how long it ran with the coolant in the
oil pan? If you think it was a while, you might also check the bearings while you have the pan off to make sure you didn't damage any of them due to
oil dilution.
By the way, not trying to muddy the waters, but your aluminum heads weren't made by Dove, were they? If so, the possibility of a transfer via the heads may be a bit higher than otherwise. See the thread:
Dove Performance sucks
earlier in the FE forum. Hopefully, though, it's just a gasket.
If all else fails, you might give the folks at Southern Automotive (right down the road from you in McDonough) a call. Bill or Louis might be able to offer some other suggestions. Keep us informed.