As others have said, hire it out or buy a bulldozer. Another great idea is just to rent one.
I am a bulldozer kind of guy and have my own so I am biased, but I could not farm without one. I have a bit more than 100 acres, but its tractive effort is 19000 pounds and sips diesel with its 43 hp engine. My Kubota, sips even less, but its tractive effort is only 2700 pounds with its 27 hp engine. There is no comparison between the two, but work well together. The tractor is fast and nimble, where as the dozer is slow and power. The difference really though is in how they are designed.
A tractor pulls...
A bulldozer pushes...
In swale building, a bulldozer with a six way blade is a dream. The sizes you listed are pretty big and not really required. A D-4 Cat or John Deere 450 are plenty big enough. You need the big ones for stump pushing which requires weight to literally hold the tracks to the ground. With swale building, a smaller dozer is fine. In fact they are often better because they have a 6 way blade. It is incredibly useful, BUT weaker then a 4 way blade. That is why on the bigger machines they only have 4 ways. You NEED a 6 way!
I built swales in a 40 acre field this summer, doing about 1/2 a mile in 4 hours with a small John Deere 6 way dozer. I use the bigger dozer for the land clearing I do, which as I said, takes 180 hp and far more weight.
I doubt you won't find further uses for the machine, but if you don't, sell it and you'll be further ahead. Just be sure to check the dozer well before you buy it as they are maintenance heavy. There is no such thing as a cheap crawler; you'll pay up front, or in undercarriage parts. But a good used smaller sized dozer in really good shape should only set you back $15,000-10,000 dollars. They are worth every penny of it.
