Just to be clear... this Burro
uses a rubber torsion suspension, right? The independent arms move straight up and down, which makes clearance an easier thing to determine (with leaf springs, the wheels tilt in the wheel wells and can rub where you don't expect). The arms will flex a bit side-to-side, since they are mounted in rubber, but it can't be very much.
If you do find you need to space them out, don't go overboard with the spacers. The axle
is designed to have the wheels basically centred on the hub faces, in the middle of the bearings. It's hard on the bearings to shift the wheels out too far, although 1/2" beyond stock should be fine.