Dunno if the SD or Adv lower lug is removable(edit: doesn't show seperate part number in parts fiche). They are on the KTM450sm and xc suspension, which I have swapped before.
I am running the 950SM. It is modified: the radial 4piston caliper on one side with the extended mounting bolts and bushings for the SD, and a single 320mm rotor from the KTM450 SM(same size as SD) on an Adv hub and 19" rim. Still one finger stopping with the remote resevoir Mastercyl and lever I matched up, but easier to modulate than with the factory mastercyl pushing both big calipers and 310 rotors.
I don't think you will be able to fit the radial mount calipers with 21" wheels. They are large and wide.
I had to grind the casting seam on the back of the caliper to clear the stainless Buchanan spokes with my 19" wheel.
You are talking about building a stable gravel blaster, however, and should consider the 19" front, like a flat tracker, an option for your design. The added road handling would be priceless with the right suspension.