Low mileage big block, locked up

Professional built big block, low mileage, driven very little, backfired then locked up. Pulled starter it's o.k., put a breaker bar on the crank, engine will not move in either direction. Where would you go from here?

I'd pull the pan and start looking at the bottoms of bores for signs of scuffing or seizure, then loosen rod and main caps as necessary. I'd also avoid using a breaker bar on the crank bolt, and use a crank snout socket or flywheel turner instead.

Did the engine builder provide a build spec sheet? If so, you might want to review the clearances and focus on any potentially tight holes/skirts/ring gaps, or look at bearings with less than 0.0012 oil clearance.

I'd pull the pan and start looking at the bottoms of bores for signs of scuffing or seizure, then loosen rod and main caps as necessary. I'd also avoid using a breaker bar on the crank bolt, and use a crank snout socket or flywheel turner instead.

Did the engine builder provide a build spec sheet? If so, you might want to review the clearances and focus on any potentially tight holes/skirts/ring gaps, or look at bearings with less than 0.0012 oil clearance.

Thanks for your reply, no spec sheet provided, but I will pull the pan. I did pull the valve covers, hoping for an upper problem, but nothing obvious. I'm not familier with a snout tool, but suppose I can get one from Snap On.