[AZKen]

K-BO: Same procedure for any motor. I don't know if it's spark or fuel with no info. I assume no fire means absolutely does not have any combustion noise and no backfire thru carb. I don't know if you just did work on it. I don't know if it was running yesterday. I don't know if it ever ran. If you are sure it's not fuel by putting thimble full of gas in throat of carb and cranking, then start checking spark at spark plugs, no spark? work backwards to dizzy, see of points open and close to begin with. Key on, points closed, open them and see if there is a slight spark, no spark? take center wire off dizzy and turn motor over and see if that sparks to ground. Report what you find and give a detailed history of your truck motor and your efforts.
Advise if you do not know how to do these test, no problem. Don't know your experience. An ice pick voltage tester would be very helpful.
The spark plug test is first, with plug wire attached and threads touching motor in a clean metal spot. See if the plug sparks when someone cranks motor. Then we can move forward slowly and methodically. If you want help, please monitor your post often.
GMC is not a C-10 (as in your truck data/profile), that's Chevy. GMC still called their models "Series" and in 68 were 1500, 2500.... with a prefix of CE, CM, CS and the "K's". They had trim, drivetrain and motor options in those models. They also had the Sierra and Grand Sierra package. But that is all off topic. Does anyone still know what a thimble full is? OK, how about a straw full, or half a shot glass. I.E. "some gas enough to fire but not too much to flood".