Has it ever run since the rebuild? If not, remove the oil pan and loosen all the main bearing bolts one half turn. Now does it turn? If not loosen all the rod bearing nuts one half turn. All the connecting rod caps and all the main bearing caps will only go on one way. If any are accidentally put on backwards, or in the wrong place, it will freeze up like that. There is a way to determine this. The main bearing caps are often numbered. The thing to remember is, the bearing tangs always go on the same side. Same for the rods. Sometimes they are numbered on the cap and the rod. If so, good. Make sure the bearing tangs are all on the same side.