Spacing headset races - is this plausible?

The problem - The drilling for the front brake is too close to the bottom bearing race to allow the caliper or nut to fit on the back (but only by a few mm.) One thing which would sort my problem would be to space out the rear bearing race, so that instead of lying flush to the fork crown as intended, it is raised by the few mm required for clearance (there is enough steerer length that this would not affect the top.). Does anyone know if such a thing might be commercially available, and if not whether there's any obvious bodge that could be done?

Surely easier to put spacers on the brake bolt?

Since you say it's a nut on the back of the bolt I presume this is an old bike and probably not a track fork that's been badly drilled? If so then surely something has been on there before?

Surely easier to put spacers on the brake bolt?

↑This

Putting a spacer under the crown race is unlikely to be feasible, since the steerer has a slightly larger diameter at the fork crown to provide the necessary interference fit, and this rarely extends any further than is necessary to take the crown race.

Yes it is an old bike, a Major Nichols, but I want to run dual pivot calipers as I feel more comfortable with them. I'm likely to get a Tektro R536 standard-nutted front, but does anyone know how long the threaded spindle is, as I need enough length to get through the fork and have maybe 3 or 4 mm of filed washer on the back. Also, the cylindrical section on the back of the caliper that mates up to the fork crown often has two flats on it, but they are not usually oriented to be exactly vertical. This would be very helpful as it would probably solve my clearance issue on the brake side. How feasible is it to loosen off the arms and rotate the assembly to that the flats point directly up/down?

Get a hexagonal threaded spacer modified to clear the race.