I have a bbc with approx 450 H.P. I am rigging the boat and want to know what GPH should I be looking for. Do I need to run a regulator? Does the pump need to be mounted before or after the water seperator? Thanks for the help guys!

Your best bet is to go with a good marine mechanical pump.
If you need an electric, I've had good luck with the Mallory marine 110gph on 500hp motors. No regulator needed with it.
The fuel water separator is not rated for pressurized fuel. Mount it before the pump.
Also mount the pump as low and as close as possible to the fuel tank.