There will be a bit more "meat" in the caliper, than thckness on the sde rim face!
Willwood have technical drawings f their range, that shows, to some extent, where you could posiibly machine it, bearing in mind the caliper halves securing bolts, and where it fouls.
In our case here, t was just up from the bleed nipples top and bottom.
When you look at where the piston bores are, where the bleed outer threaded tube screws in,, and depending on the caliper mdel, you MAY be able to take some off, but only do the immediate area.
Ther option, f ypu have flares, is longer studs, and use a slip on type spacer, that s then mechancally fastened to either the hub, or backface of rim. Countersunk machine screw into a tapped M5 hole, s sufficient.