General rule of thumb is five PTO horsepower per foot of cutting width. You could probably get away with it if you took it easy but a four foot cutter would probably be more appropriate. Your tractor has the lifting capacity but unless you have a loader it may get a bit light in the front end. You can get the numbers off of Tractor Data: http://www.tractordata.com/farm-tractors/001/2/7/1275-kubota-b9200.html
Oops - my bad. I saw King Kutter and thought mower as that's what I have. Box blade should be okay if you don't dig so deep the tractor stops.