We put a 70 short box sweppy on a 78 short box frame, it fit like it was made for it. I think we trimmed off the top of the frame above the rear axle on the 78 frame, and had to relocate the front 2 box mounts and the radiator support brackets.
That tells me the Sweepy sheet metal should fit pretty well on up to the 93 frame. I don't know how the 98 frame compares to the 73-93 frame. Gene