My L3000 has about the same hp and has a few more lbs but for the most part about the same tractor. I have a Bushhog BX650 5 1/2' box and when really moving dirt it's more than my tractor can handle without lifting. I don't lack hp as bad as lacking traction most of the time. Then again in the right soil conditions when the tires are getting a good bite I can run out of power.
That said, I agree with MtnViewRanch. In most cases you can handle a 6' but don't be shocked when you get a full load of dirt it will either spin out or lug down to the point you have to lift up and shed some of the load. You will have times that 6' is to big. JMHO