I run a stock drive on my HV pumps with no problems at all. And go ahead and try installing the new pump with the drive in place. Spin the pump back and forth as you slide it up into place so that the hex can align. If it fights you, you can always bump the engine over a bit to help align the hex.

Question; Does the HV pump have that much more resistance to turning than the std pump? It would seem that the pressure would impact the resistance more. At say 60# pressure, how much harder do you think it would be to spin an HV pump over a std pump?