1990 Chevy 1500 Pickup 5.0 Starting Problem.

You truck's security system may be preventing you from starting it up. Do you have a resistor chip in your key? The VATS (vehicle anti-theft system) has become a pain for owners of older Chevys. If you have a resistor in your key, wipe off the key with a cloth and see if that gets the truck to start. If not, try your backup key (if you have one).

If that doesn't work, you may have a big problem. There are two very thin wires soldered to the ignition lock cylinder. Those wires complete the VATS circuit when you place the key in the lock cylinder. Unfortunately, Chevy cheaped out and made them WAY too flimsy. They break or come undone from the lock cylinder and then you're up the creek. It will not start, period. 

The solution to that problem is to remove the airbag, steering wheel and turn signal switch to gain access to the wires. Then you will have to re-solder (or replace) those wires to get the VATS working again. There may be a way to bypass the VATS setup, but I am not familiar with any way to do that. 

Sounds like defective fuel pump.