Engine of barn find-steps to evaluate/save
OK,..... so it's not the starter, or frozen valves, & You can rotate the mill "several times" clockwise......but not once counterclockwise.
Questions;
1) If You stop turning the engine in a different position, does the amount of rotation before locking change, or is it the same each time?
2) You stated it turned smoothly & quietly, so no light scratching/scraping, clinking/ticks at all..correct?
3) When it stops w/a clunk, turning slowly, does it feel like it binds up..or an abrupt/rock hard stop?
At this point, I'd pull the trans shield, & give one of the flexplate ears/screw/converter lugs a squirt of bright paint to line it back up. Then take the converter screws out & push the converter back & make sure it's free from the crank, then repeat the crank roll & see what You've got.