I just replaced my starter motor

The fuel pressure issue may be the pump or the relay. When you turn the key to the "RUN" position have someone lsiten on the right side of the truck next to the fuel tank. When the key is turned to "RUN" it primes the fuel system by running the pump for a few seconds. You can usually hear the whirring noise of the pump if you stand near it. This indicates that the relay is working okay and the pump is at least turning. If you don't hear the wirring noise the relay or the pump is bad and there is no fuel being pumped at all.
As for the starter I am a litttle confused. You said it won't cranks over but then it cranksover until you disconnect the battery with the key removed. It sounds like you have an issue with the starter switch and or the solenoid. The other possibility is that the starter relay is stuck in the on position.