Looked in the service manual for bolt specs. They list M8 x 1.25 on the threads. Looks like it is 35mm in length. While these triple mount clamps are cool and all, I think the issue you will encounter is that there is a plate on each side of the upper triple clamp that Yamaha uses to mount the front plate/headlight. In addition, on the left side, the upper triple clamp also holds the nylon guide that routes the front brake line.
I suggest you take a look at the parts schematic under "steering" and "electrical 1" and you can see these things that I mention. This is why I didnt use the triple clamp mounts for the WR450F. Most people go with the Cycra CRM (center reach mount) or with something like this BRP hand guard mount (which is what I did) that also is a handlebar clamp:
Im sure some guys have figured out how to deal with these issues with the triple clamp mounting, but it didnt look easy to me and I was concerned I would come up with a half assed solution, so I just went another route (BRP handguard mounts with Cycra CRM hand guards).