If you put a floating spacer in to get the clearance, say 5mm, then you wont have a lot of purchase on the nut...maybe 4 to 5 turns....not enough to be safe so you will need longer studs.
Second option is a bolt on spacer but the minimum thickness is 15mm (i could be wrong) and this may be too much and make the tyre hit the guard.
As mikey said, the offset of the rim will be the deciding factor as to what you need to do.