It sounds to me like the clutch for some reason isn't disengaging. If you crawl under there and remove the shroud (and really there should be two but occasionally the vertical one may be missing), you can have somebody push in on the pedal while you watch and you should be able to see the pressure plate pull away from the clutch plate leaving it relatively free to move. Or another test: Can you put on the parking brake, put the transmission in first gear, push in on the clutch and then start the engine? Does it feel like the truck wants to travel? Important note: be ready to kill the engine if the truck starts moving. This is the least safe of the two options I've given you. There may be others, but at any rate I would say your clutch isn't letting go...
I've rebuilt these transmissions, too. They were very simple gearboxes compared to some later ones. Usually I found problems with first gear (people impatiently trying to force it into gear coming back from second gear or using first as a downhill engine brake...that always caused odd gear wear patterns and ruined rollers), always with the roller bearings, sometimes with reverse and that was about it. First was not synchronized as you probably remember but if the clutch lets go, it will usually go into first without trouble. Going into second before putting it in first will usually stop things from spinning as much.
Jon
