I bought a new Chevy truck in 91, and didn't like the way it pulled the inlaws camper. Weight rating was fine, WD hitch helped, but still didn't feel right. I asked Chevy, and they were ok with it, as long as I didn't exceed limits. So I installed a set of coil helper springs above the axle on rear leaf springs. It kept the truck from squatting so much when I went to max tongue weight.
Adding spacers, etc to the box is asking for trouble, and it will only take ONCE, for you to get a rear heavy load up to speed and when you let off the gas to level out, it starts fish tailing. I put a new 80 Chevy with less than 5k miles on it in a ditch, jackknifed, both drivers side tires flat from rolling off the bead. Haven't forgotten that ride! Give me tongue weight!
Another idea, is put a broomstick thru a pocket or weld a piece of pipe (hose clamp it?) and paint the broomstick red and green to match just how much tongue weight you can stand. When it gets to the red, more goes on the rear, not enough on the front, put more up front.
I probably still have those helper springs if you need a visual. The first time we tried it, we added Ford Fiesta rear coil springs to a 84 Ford pickup. They were too tall, but the guy said leave them, so we did. They shouldn't support any weight until the leaf springs start to straighten.
David from jax