Stock Rustler width (at least on the one I have) is 12.25 inches.
I've got one modded out to 13 inches in the front and 13.5 inches in the rear. This is not trivial or for the faint of heart and requires a bunch of non-traxxas parts.
Bandit front A-Arms (because of the 1/8" outer hinge pin) Losi XXX-T spindles and spindle carriers, AD2 front axles, T3 front wheels installed inside out (to get more offset). This leaves you with bearings in the wheels and is not hard on the bearings.
XXX-T rear hub carrier narrowed to fit in the traxxas A-Arm, XXX-T universal style axle and cross pin to fit in the outer end of the traxxas slider. T3 rear wheels (more offset than Losi). Bearings are pushed way out in the rear by the XXX-T carriers so no more stress on them than a XXX-T or T3.
You could get a little more width in the front with regular Rustler A-arms but there is a hinge pin size mismatch you would have to account for in the XXX-T front carrier.
Losi makes XXX-T rear carriers with different amounts of toe that you could use instead of the standard ones to get some rear toe on the modded rustler.
Obviously you will need different turnbuckles to accommodate the greater widths, not to mention 3/16X3/8 bearings for the front wheels and rear carriers.