well, from what I know, 3/4 tons can have either 52 or 56 inch springs-most are 52 inch, I have only seen one 3/4 ton with 56 inch springs
if you just want a better rated spring, you should just be able to source springs with more leaves. , the fronts shouldn't change too much if you have 3 leaves, unless you want 4 leaves(really only need on a diesel IMO) for the rear, figure another 500 lbs per leaf, so if currently a 4/1 spring, a 5/1 will get a 1000 pound increase, I believe a 8/1 is the top for 3/4 ton, 3500LB capacity
73-87 springs are all the same, in same truck rating(1/2, 3/4 and 1 ton), and like I said, 1/2 and 3/4 should be 52 inch springs, except in some cases(heavy duty, those should be the 56 same as 1 tons
hopefully this helps clear it up a little
hey Magic1, how was shipping from SD?