We just bumped into this problem also (pun intended). We couldn't get the distributor to drop in, and when we looked down the into the hole we could see the oil pump was WAY out of alignment. My builder took the pan off to discover the oil pump interfering with one of the ARP main studs. He resolved the problem using all the options noted above. He shortened the stud, ground a little off of the nut and a little off the pump. He took a bunch of pictures that he wants me to post here. I'll post them as soon as I get them.