It is a great looking ride for sure!
Let's see now: that is a basic Hotchkiss type live axle, simple but very strong and effective. The springs have 8 leaves in two groups for a progressive rate. Counting from the top (longer to shorter) the first one has the shackle eyelets, then the next four are used all the time, and the last 3 come into play when the load compresses the spring to bring them into play.
Chaging this type is relatively easy. You can experiment with removing the leaves, except the first with the shackes and the last shortest one, as long as the thickness of the spring at the U-bolt mounting is kept the same with mean of a simple steel block of the correct thickness with a hole for the locating center bolt. This spacer need not be long, just the correct thickness and can be added between the top leaf and the bottom of the axle to keep the ride height the same as stock.
I agree with you with starting with the #3 leaf.
This will reduce the jounce at the back end, and you will probably need some weight still, but it will be worlds better than just adding weight, because the addition of the weight will require stronger dampening by the shocks to be effective.
Adding a locker will improve traction and will be a good addition if you can find a suitable unit. There is enough space for an air-bladder kit to be added too. Please let me know if I can help source any parts that you might decide on, after wise people (unlike me ) chime in too.
BTW, what are the brackets along the rear axle that are empty? That looks like the positioning of the handbrake assembly cables. Do you have a functioning handbrake in the truck?