When you fitted the oil pump did you get it centrel on the crank you didnt make it line up with the bottom of the block did you? remove pump and try the crank then, most people try to make the oil pump line up with the block fluch at the sump side it may look right but the pump will be offset on the crank and have a tight spot ive seen pumps shatter this way read the haynes manual its there about this point.
All i can think of unless the caps are distortin the block when there tightened up ?
Best of luck,its best to bore the block with the main caps torqued up also,so im told anyways lol.