If you are using a non-floating caliper, like a 4-piston Brembo, Yamaha"Blue Spot" or similar, the disc will have to be close to the fork slider.
On my 1977 XS, I ordered a custom ISR 320 mm disc with 23 mm offset. This is effectively the same as the popular 320 mm Brembo from Ducati (10 mm offset) plus a 13 mm spacer. In any case, the disc floater buttons will have 1-2 mm clearance to the fork slider.
I got a custom made disc to avoid the weight and added complication of a spacer and longer bolts.