If the engine will start and idle after you put the transmission in gear and start it, it's probably time for a clutch change. The clutch disc has come delaminated and it's not possible for the pressure plate to retract far enough to keep it from dragging and making the gears clash. A clutch disc stuck to the flywheel would break free the first time the engine was started in gear with the pedal depressed. Take the dust cover off the bottom of the bell housing, and look for evidence of contamination or separation of the clutch material. If the truck is original, you'll have to deal with the closed driveline, so a clutch change can be a pretty difficult project.
Jerry

"It is better to be silent and be thought a fool than to speak and eliminate all doubt!" - Abraham Lincoln
Cringe and wail in fear, Eloi- - - - -we Morlocks are on the hunt!
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