When I installed my Triumph Slants and brackets the supplied bolts were exactly the same as the existing fender bolts. Wouldn't have mattered so I used the originals.
As long as you can fully tighten the 4 new bolts you should be OK.
Put your hand up under the fender to make sure there is no interference with the tire.
If there is an excess sticking out then you might want to try the original bolts before buying new ones.
