Cab & Chasis 14 bolt is aprox 63" WMS-WMS. DRW D70 HD is around 74" WMS-WMS. I would guess the late model 14 bolts from DRW pick-ups is similiar in width, Thise came under 88 & up new body style trucks.
Lots of people have done what you are wanting to do, they just are not many on this board.
I too think you should look into larger than 14 bolt axles. I am thinking Rockwell 2.5 or 5 tons would be the ticket.