Your basically bone stock GMC would be a positive ground system - note that when you install the battery.

Be sure to polarize your generator/regulator - check your GMC Shop/Maintenance Manual

Here are notes/information from a Chevrolet Manual (bottom right on each page):

http://chevy.oldcarmanualproject.com/shop/1948_51truck/51ctsm1207.htm
http://chevy.oldcarmanualproject.com/shop/1948_51truck/51ctsm1213.htm