This is basic stuff off the top of my head.
With the truck parked and engine off. Chock the wheels. Have someone push in the clutch pedal while you look at the clutch arm.
If the arm doesn't move much or at all, check that the jam nut is not loose. If that looks okay and there is plenty of movement, it's internal.
Take it to the repair shop. If you want to do it yourself, look here:
http://chevy.oldcarmanualproject.com/shop/1948_51truck/index.htm
