I found this. Your end play may be too much.
CAM "Before installing the pistons and rods, check the end play in the crankshaft by forcing the crankshaft all the way forward or aft, then slipping a feeler gauge between the bearing end cap and the base of the crankshaft counterweight. The reading should be between .003-inch and .009-inch. If it is not within tolerances, replace the rear intermediate main shells and check again. Also, on the Chevy, check the lash between the timing gears with a feeler gauge. It should read between .004-inch and .006-inch. "
