Used engine will not crank over?

I purchased a good used 5.0 ford engine from a wrecking yard and changed out some of the gaskets. It turned over all the way when I first had it. I dropped the engine into my truck and began trying to line up the flywheel to connect with the tranny. We where hand cranking the engine with a breaker bar when the engine stoped cranking, it won t go past a certain point. We tried cranking it back counter clockwise but it gets stuck at the same point. Any ideas what this could be? Some one said I probably dropped a bolt when changing out the oil pan but I don t think I did. If you ve came across this problem before I d appreciate the help. I d hate to have to pull the engine out again because it s a pain in the ***.

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