Unfortunately, I suggest a teardown. Go in stages, because if you find it, you may be able to correct that one problem, depending on what it is.

I don't think a complete rebuild will be necessary again, because I don't believe there's any metal in the oil. It never had a chance to spin a bearing while the engine was running.

But first, you have to take the starter out anyway to change the motor. So take the darn thing out and see if your motor will turn...just a hunch.

I think I'd probably start by checking out the bearings. Main and rod. Plastigauge them while you're at it to make sure the clearances are within spec. I'd do the mains first, and then get the caps back on if that looks good.
Next, I'd do the rod bearings. But I would take them loose one at a time and try and pry the crank to see if it moves. You might just find the single culprit this way, and if it turns out to be one of the connecting rods or piston wrist pin, then fix that one thing and put it back together. Each time a connecting rod is removed from around the crank, see if you can move that back and forth in its bore. If you can, the wrist pin is not seized.

If you get down to all the rods taken off, plastigauged, and put back on, and you still can't turn the crank, then maybe start looking at the cam. These stovebolts have direct gear-driven cams. It might not hurt to make sure some errant piece of foreign object or metal did not get stuck in the cam gear and stop it from turning. A stupid little thing like that would do it. You could check that first if you'd like, just to eliminate doing the rest of the motor.

Good luck. My 235 runs great, right up until one of the cylinders gets too much water in it to run anymore. It's cracked. So I'm going through a motor change too.

52 GMC 3/4 ton pickup
68 Big Block Vette
68 455 Firebird