how much fuel pressure do i need?

the stock 2.3 ranger setup had two fuel pumps, one in the tank that ran 6psi and one booster on the fram rail that ran about 40 or so. years ago when i put a 5.0 in the thing, i carbed it and just used only the 6psi in tank unit with no return. now that i am going to inject it i plan to keep using the in tank 6psi unit and use another inline as a booster as the factory did (mainly to avoid the hassle of dropping the tank/pulling the bed, and running high pressure fuel line of that length). i want to mount an adequately rated pump under the hood somewhere and just run high pressure hose from it to the intake. i just dont know what psi rating to look for in a pump. various 5.0 models i look at call for anywhere from 40-65. what happens if i put on a 65psi unit and it only needs 40? i am assuming one of two things. one, the regulator sends the overage back to the tank, or two, the pump itself bleeds off the pressure that the regulator wont let past itself.

so, if i put on a 65 psi unit, what can i expect? what about a 40psi unit? i tend to think either will work.