FYI / Update:
After letting the mystery marvel oil sit in the cylinders for a month I came back and gave 'er another try with the breaker bar at the crank pulley. Nothing; she wouldn't budge. So, with help, I yanked the 428 & transmission out of the car, separated the transmission from the block and put the engine on a stand. While trying to remove the flywheel, we noticed the engine now turns over... with relative ease! Turns out, it's the transmission that won't turn.... but the 428 turns over just fine. It has excellent compression on all cylinders except 2 & 8. So weird, I've never encountered a situation with an automatic trans wherein the transmission could keep you from turing the engine over by hand. Shouldn't the torque converter always disconnect the engine and trans? I don't understand this at all.... but at least I now know the engine is in much better shape than I thought.
If anyone understands how a locked-up automatic trans could prohibit an engine from turning over, please, please educate me. :-)