I think what you will find the issue is what will fit on your rims more than what will fit under your fenders.
With the big-6 lug pattern split rims you are limited in your tire sizes and choices.
Unless your tires are rotten I would run what you have out and wait until you have to purchase new rubber.
By that time you will have saved enough $$$ to either swap to modern axles with the common 8-lug pattern or you will have saved the same $$$ to purchase new big-6 tubeless rims that will let you mount a variety of tire sizes you can't mount now.
By the way, really nice looking rig!