Is it that hard to find a F-650/750 with air rear susp? They are pretty popular trucks, not just for box trucks but rollbacks, flatbeds/stakebeds, dump, and whatever else the upfitter has added to the cab and chassis.
Even with air rear, the trucks have leaf spring fronts. Only option that may exist for front is an aftermarket company like Kelderman. Kelderman is also a source for your air conversion. Just looked up, and only a rear is available from Kelderman, and it is intended as a supplement to the stock leafs; mainly being air ride for low loads and then leafs for the heavier: