In the cold, hard, light of day there’s another possibility for the brakes. I could tap the caliper mounts in the uprights, as shown here, for say, M12x1.5 and I could then fasten the caliper mounting blocks directly into the upright without needing a bolt head on outside in the way of the disc. That way I could use the sort of discs that I’ve got on the back of the Fury and I’d be sure they’d fit under the wheels.
This is essentially how the front brakes on the Fury mount to the uprights (ex mk II Escort in that case) so it must be possible to make it work, especially with the addition of copious amounts of thread lock. Certainly seems a decent idea…