I used a 6' tiller on a 32 HP tractor for a couple seasons. At least it WAS 32 HP when it was built, some 40-sumpthin years earlier. It was on my old MAssey Ferguson 50. Gas engine that had over 10,000 hours on it too. Handled the chore just fine. Trick is to go slow, take shallow cuts if you're in hard ground, and don't try to till rocks. (I did, and just about bounced the ol' Massey off the ground a few times)
I use it routinely on a 38 HP MF 150 nowdays.