I have often had the same problem with my '66 3 speed. In my case I would have to put the e-brake on, press the clutch (to make sure the weight of the truck was not on the transmission.) Open the hood and push down on the rod nearest to the firewall and pull up on the other one, then the truck would begin shifting again. Like Red58 said, it was from making a poor shift, in my case if you just kinda let the gear shift slip from first to second gear without firmly holding it and placing it into gear.
