slow to respond, but a dually stock runs a 225/75R16. TINY tire. You can go up to a 235/85R16 without the need for rear spacers between the duals, or you can go for spacers and run a 255/85R16. That is what my 98 had, what my 06 has, and what @83GMCK2500 has.
I calculated it out, me switching...