To get it to work, you "should" (my memory is a bit fuzzy) just need to power the relay, like the computer did, you can use the in tank pump, but still need a regulator before the carb. The in tank pumps out out more than the carb can handle alone, like 20 -35 psi and all you need is 8-10 at most, more likely 5-7. You will have to remove and/or bypass the frame mounted high pressure pump.
I plan on using a regulator, but if I'm not mistaken I think the in-tank pump puts out somewhere around 8psi. They are used to get the fuel to the high pressure pump on the frame rail. These trucks have 3 pumps, 1 in each tank and then 1 on the frame rail. As far as I can tell from the wiring diagram the power comes from the ECM to the inertia shut off to the tank selector switch to the pump. I figured if I can supply a switched power source before the inertia switch everything should work just fine. Thanks for your help guys