Here is the best way i have found to set the distributor and have the timing right on the money with out having to use a timing light.
1. pull the number one spark plug.
2. have some one stick there finger in the spark plug hole to seal it off.
3. turn the crank shaft till they fell compression pushing there finger out of the hole.
4. turn the crank till you have the piston all the way up to top dead center.
5. if your truck has a timing tab set the timing to 12 degrees before top dead center" you will have to turn the crank backwards just a little bit to get the timing set to 12 degrees"
6. With your truck having a vortec engine you will have to turn the distributor cap upside down and see where the number one plug wire tang is on the inside of the cap.
with a magic marker mark the cap on the out side of the cap so you can see it when you turn the cap back over.
7. pull the distributor back out and turn the rotor button so it is just before the mark on the cap for number one if you where to put the cap back onto the distributor.
8. put the distributor back into the engine and see if the rotor button lines up with your mark. if it does not pull it back out and turn the rotor button just a little bit till it is right.
9. if the distributor does not mesh with the oil pump rod use a long flat head screw driver to turn the oil pump shaft till the distributor will fall all the way down.
10. line the pick up coil with the star on the main shaft of the distributor and tighten the hold down. put the cap back on and put the plug back in. the truck should start right up. If you have tried to start it and the timing was off the cylinders may have gas in them so I would hold the gas pedal to the floor before trying to start it and hold it there till it starts to start.