If you determine the oil pump shaft is too long, you can take it to a machine shop and anybody that knows how to run a valve grinder can chuck it up and knock a little off the end of the male "slot". I would also double check that the oil pump drive gear is all the way down in the bushing as far as it will go. Should be pretty easy to determine if the distributor is being held up out of the sealing groove by using a mirror and looking at the distributor flange to see if it is flush against the block.