My bet is a main bearing spun. My limited experience with rod bearings failing is they spin in the rod and usually don't lock the engine down. But they make a terrible knocking sound until they come out. One engine broke a rod in half when the rod bearing spun and locked down on the crank.
But a main bearing usually locks the engine up (especially at a low speed) and about all you hear is a "thunk" and then no more turning the engine. Had a 383 Dodge engine one time lock up as I was traveling 60 mph down the road. Main had spun and locked the engine down.
Hot Rod Lincoln gave a pretty easy way to see..... loosen the main cap one at a time until you can turn the crank....then use a prybar on the flywheel teeth to see if the crank will turn..... I bet you find it soon. Listen to Hot Rod, he's mighty sharp.
