Dually with a step side box: No. They came cab and chassis, flatdeck, grain box or single wheel long box. No dually fenders.
They can be built though.
A 3/4 ton box might fit on the frame but you'd need spacers over the duals. I'm not 100% sure of the wheel base difference lining up properly. A 1 ton box has extra length behind the rear fender but I am not 100% sure if the 3/4 has a shorter distance between the front of the rear fender and the box front. If the 3/4 has a shorter box front, then things won't line up easily for you (ie bolt in).
Check out my build. It may address some ideas you're having about a dually box. This was a PILE of work.https://www.stovebolt.com/ubbthread...new-9ft-bed-progress-1ton-gmc-build.html
Plowboy on this site also has a very slick dually box that is a little different than mine.
Based on reproduction parts only listing splash aprons for 3/4 , I would assume that the length of the front of the 3/4 box is shorter than the 1 ton, otherwise they would list the part as fitting both 3/4 and 1 ton. This would mean that simply bolting it down would not work.
But anything is possible if you're inclined and have the skills to make one fit.