Not that it matters, but I was thinking the 8N/9N tractors were under 20hp? 15-18hp? I could be wrong here.
Either way, I have a 30hp diesel Kioti and 6' is a lot of bush hog to pull. We run a 78" Mott flail mower. If you stay on top of the fields it is fine. Let them get ahead of you and I have to really slow down or it works that tractor hard. I just listen to the flail mower. It slows down, I drop a gear. It can't keep up, I drop a gear. If it is tall and wet, low range 1 or 2 is not unheard of. Seems 5' is what most guys run on the size tractor you have. Of course, like anything else, just feed it what it can handle and you will get by with the 6'. If you got it for a good deal, then I would try like heck to make it work. lol