MtnBoy,
If you measure the fork tubes, and the micrometer or caliper says 34.993, then the surely are 35 mm forks. I believe the 34 mm forks had the caliper on the RH side, and in front of the fork slider. The 35 mm forks had the caliper(s) behind the sliders. In any case, no Brembo 4-piston caliper will bolt up tho the stock KYB/ Yamaha fork slider.
Just call or email whoever you are ordering from, and tell them the caliper lug C-C (spacing), distance from wheel axle center, and the brake disc diameter and offset.
If you plan on using a Brembo 4-piston caliper, the disc needs to have 22-23 mm offset in order to clear the spokes, when using a 320 mm OD disc. This means the disc only has approx. 2 mm clearance to the fork slider.
Good luck with your brake upgrade, that is one of the areas on the XS that can be improved a lot!