new head gaskets & turned crank without marking distributor. can`t find timing

If your truck has the the flat style vortec cap there should be two triangular tabs inside the distributor housing. one has a number 6 and the other has a number 8. Bring the engine up to top dead center on the compression stroke on the number one cylinder. Point the tip of the rotor exactly to the number 8 if a v8 or number 6 if a v6. turn the oil pump shaft if with a long screwdriver if needed to make it drop and seat correctly.