I quickly checked the body dimensions page for a 56, there is no 57 truck manual listed, and it gives the height from ground to top of frame at the rear, standard, without body-I assume that would be without the box, it is 25.6". I assume they would have the dimension at the front be the same as there is no dimension shown for the front of the frame. meaning the truck frame was engineered to be siting level at the standard height. from the top of the frame to the top of the front window opening is 44.87". I know, weird way to do dimensions but thats what the diagram shows.
dunno if that helps but maybe use the frame to ground dimension and see how yours stacks up. that is with the stock tires that would be 6.70 x 15
https://www.cokertire.com/blog/tire-...ross-reference