1998 GMC Sierra with 5.0 will not start.

If you've confirmed the spark plug wires are in the right place, that youve set number 1 cylinder (front drivers side) at TDC, confirmed the valves for number one are both closed and that the dizzy is pointing at number 1 plug wire you should be good to go.... set the rotor one tooth back and see if that will set it JUST before number 1 post. In my experiences if its set more or less dead center its too far, if anything you want it to lead into it a hair. While someone cranks the motor turn the distributor one way then the other, one way and it's hard for the motor to turn over, the other and it's cranks really quickly. Setting it so its about midway has been a reliable starting point for me in the past. Don't forget to confirm the dizzy is plugged in and that you have fuel and spark also