BH makes no difference, the centerline of the motor and the length of the clutch fork do..
With a different BH you can have a altered pivot point, but it will not change the length of the Fork..
If you do shorter, be aware that you might put too much pressure on the clutch (depress it...
Personaly, I would cut the fork down.. To the best on my knowledge there is not a shorter fork. But, since the 80 did come with a manual in other countries. Calling someone in AU to find ya one might not be a bad idea.