Cab and chassis narrow Dule axel question for shortbox 4x4 project

[Tex_50613]

I am starting a shortbox towing rig project starting with a 84 GMC 3/4ton 4x4 shortbox according to the tital and the 8 lug axels (fraim mic's out thicker than the 79 3/4 ton fraim under my mudder also) and happened to see something that gave me a idea, but the question is How wide is the narrow 14 bolt cab and chassis dule axel? I happened to see one under a 75-76 chevy wrecker a little while back and the outer dule didnt look any wider than where a stock non dule tire would sit but befor I waist my time to find one I would like to know if it all the tires would fit under a step side box if I tubbed it wile keeping the stock overall with?
here's a pic of the desaster I am starting with


the intended end result is a unique 4x4 5thwheel puller to haul a 38' flatbed with 2 mud trucks on it, in the works is a Diesel motor, 4 speed + OD manual trans, np205 t-case, 1 ton suspention up front, simi air springs with heavy 4 link rear, and hopefully a dule rear axel without having to widen the good stepside box I have and would like to use on the truck