don't know about the disc / drum bed issue... however do know some other things you might want to hear... sorry if it redundant to you.
the axles on a Dually are about 4" narrower that non dually, the beds are cut out to have tire /wheel clearance for the inner tire, and the dually fender covers the cut out, the dually factory fenders are about $1000. 00 ea new from dealer.
so if you can work it out with Mark. . that would be the way to go IMHO. . and if you wanted disc brakes the swap it out later as the drum brakes are not that bad... you can lock them up on either so the stopping power is about the same. . just easier to change the pucks on disc's... . oooops reread Marks specs. . his might be disc also