1988 Chrysler Conquest engine seizure

I had a longblock rebuilt at at a locaL shop, after putting it in and running for 20 minutes and shutting it off, it wouldn't turn. The shop told me it was a problem with the crank kit and rod bearing in clearances, mains were fine, reground the it and re installed it for me. After putting the engine back in it was The same again. The engine fired right up, great oil pressure, ran perfectly cool. This time it was run for an hour and couldn't have been happier. Any ideas what the cause may be? THANKS!