I have a '65 C20, and let me tell you, I now know where the old saying "it rides like a truck" came from!! I'm sure you'll gain a little load capacity but you'll pay for it in ride quality. Other things to consider: tires play a big role in how much a truck can carry. Also, 3/4-ton wheel bearings are heavier duty than 1/2-ton. The front end of the truck also carries load weight, so you don't want to carry so much of a load that your front suspension is compromised. Brake are another factor.
All things to think about.....
