Note in the picture that in addition to the switch on top of the starter there is also a bendix which can be replaced separately. However with the symptom you describe a good inspection of the switch is in order. Be sure the foot starter linkage is fully depressing the switch.
http://chevy.oldcarmanualproject.com/shop/1948_51truck/51ctsm1218.htm
Last edited by Forty9; 08/27/2015 9:16 PM. Reason: spelling