The early and late pumps also have different placement of the timing marks, so if the donor engine is a 74 and you are using the original harmonic balancer, the timing mark would be WAAAY off if used with an early pump.
it's the timing cover that has the marks cast into it, not the pump.
on 68 and earlier engines there was a timing tab that bolted to the front of the pump.
due to the outlet, using the later pump on the earlier cover obscures the tab or marks, so you would need to re-mark it on the other side or somewhere visible to set the timing.