When you get the 390 started, check for bubbles in the radiator. Blown head gaskets can manifest themselves in many ways, but they're all bad.
Antifreeze in the crankcase will wipe out bearings. Coolant usually shows up on the dipstick as described.
This 'black' substance you mentioned, what consistancy is it? Could it be anything BUT oil? Usually, oil floats on top. My neighbor mixed orange coolant with green and it turned to muck, too thick for the water pump, and eventually the engine overheated (even though he swears it didn't). He thought he was doing his Blazer a favor by 'giving it the more expensive, better stuff'. Turns out, this is very common with folks who just don't know what to buy and are afraid to ask.
An overheated engine usually warps the castings. A machine shop can straighten them out by milling (commonly done as part of a 'major overhaul').
I don't buy old engines to save money, but with the idea that I will overhaul them to modern standards. Remember, pre-1980 engines were made for different gasoline and flat tappet oil. The castings may be worn and inexpensive, but Magnaflux proves them to be solid (or we aren't going further). Any good parts I find in the process, are considered a bonus.
By overhauling, I leave nothing to 'chance', and my engines last, well, like modern engine performance. I expect 150K without burning any oil. - Dave