1968 318 water pump?
An easy fix for the timing mark when you're using the older style water pump with the new cover is to make a new timing tab. Just line your harmonic 0' mark with the mark on the front cover as you normally would. Then make a new indicator on the other side of the motor and align the 0' mark of the timing tape to it. It's inexpensive and you'll have the new indicator in a location that isn't hidden by your lower radiator hose and power steering.
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But in your situation you're doing the opposite. You'll be using the newer style water pump with the older front cover. I would guess that the solution could be done similarly.