They make tools to drill and tap your block straight but if you cant find one you can make one if you have access to a mill. take a piece of steel at least one inch thick and drill a hole in it thats the tap drill size,then drill another one next to it and tap that one.on the other end cut a slot that is wider than the 7/16 bolt.position the slot so it will reach any other head bolt. simply center the fixture up on the bad hole and bolt it down.using the tap drill size hole to square your hand drill,drill the bad hole to tap drill size. loosen the fixture and position the hole you tapped over the bad hole and clamp it down. screw the tap through the fixture and into the block.thats all there is to it.Bill