Small Block in an early A - how much room for a fan?
keep in mind that the early waterpump exits driver's side and the late waterpump is pass side.
this is important for radiator hook up and timing cover. you can't see the timing tab with the pass side outlet in the way. plus, the the mark on the balancer coincides with the timing cover, which not the end of the world, you can always re-mark it.
so really it's more about matching the radiator, if you already have one.