In my opinion an 18" rear with a knobbie would not fit into the SM swingarm and the 21" Adv. wheel will definately not fit on the front. The problem is that the tire would make contact with the radiator when the forks compressed.
I did use the Adv. wheel axle spacers on the rear, but the fronts had to be custom.
And no, I never considered using an Adv high exhaust on the bike because as I said at that point I bought an 950 Adv. and solved the whole problem. I have heard people talk about it, and in my opinion it is possible, someone just has to step up and do it.
Brents347 Truckee, CA.
07 Yamaha R6
07 KTM 950 Super Enduro
04 KTM 250exc- plated two-stroke