You said the truck is a 98, what year is the engine out of?

Were all of the electrical connections made?

If the engine is from a different year, did you use the sensors from the original truck engine?
That can possibly make a difference.

I believe that the fuel pump gets it's ground through the computer (JTEC, NGC).
If the computer is not getting a signal from a sensor it may not be sending the ground to turn the pump on.

A buddy found that out the hard way when he converted his truck from fuel injection to a carb.
Without all the sensors hooked up the fuel pump wouldn't get a ground and the pump wouldn't run.
He ran a separate ground and then the pump worked.