As an owner of a '68 A-body, I would like to use the short water pump and not the longer '70+ pump for more clearance for radiators and fans. 3/4" is a lot for the A-Body engine compartment.
I used that how to make some changes so I could use a factory ac water pump pulley. In addition, a parts store pump doesn't have a shoulder for the bearing, so it didn't need to be bored
If anyone decides to follow that path, I have the tool I made to press in the carbon seal available to borrow.