transmission

I was reading the "ultimate 650/750" post and I got the idea that different trannys might fit my KZ650, I have two 600ninja engines just laying around they have 6 speeds, will it fit my 650 ?, would it be an advantage ? Sorry for sounding dumb but raising kids and motorcycles didn't seem compatable, but my youngest is over the magic age so I am on the bikes again, but work in autos for my whole life so mechanical stuff is not foreign to me