Try tightening one main cap at a time, and see if one of them stiffens it up more than another. It's possible the alignment between the bores is off. Getting a block line bored is a pretty expensive proposition, but unless all the bearing bores are in alignment, it's possible to lock the crankshaft up even if indivual bearings each have the proper clearance. Also, be sure there is NO lube between the bearing shells and the block.
Jerry
