Just the fact that they are shorter will increas capacity.
What is the GVW of your truck???? 5600 or 6200.
Both of these trucks used the same rear springs that are dangerously
close to the most common 3/4 ton rear springs---6400 GVW
If you have a 5600gvw truck---change the front springs and add a sway bar (from a 3/4)----quick and easy way to get to your (close to) 3/4 capacity.
If a 6200 gvw----leave it alone---already right at 3/4 ton rating.
The 4900gvw trucks only had 3 or 4 leafs in the rear, weak front springs and no sway bars at all in 79
I cannot recall, but I thought the 3/4 rear springs were a bit wider as well?
This would be a front hangar change and rear shackle change.