Another consideration while you're messing with the pump swap: How large are your fuel lines? ALL of your fuel lines. All the pumping power in the world still won't suck a golf ball thru a garden hose any faster...
Depending on what your car was born as, most of these cars had 5/16" fuel lines in various locations between the pick up in the tank and the carb. Not to mention 45+ years of mechanical hacking history. A single inch of 5/16" line is a MASSIVE restriction in fuel flow.
3/8" should be considered a bare minimum from end-to-end, in order to maintain enough flow to feed a 400 with any mods whatsoever.
