Well, IMHO, your tractor, being 47hp, would be adequate to handle either a 60" or 72" box blade. I come from the "camp" that says your box blade does not have to be wider than your rear tires to be effective. And for around $300 and being in nice shape it would be a good buy.
My tractor is 62hp and I have a 72" roll over box blade that works just fine. My rear wheel width(outside to outside) is 80".
Buy the 60" unit and try it. If you do find that a wider unit would be better - you can always sell it for the $300 and loose nothing.
You could be waiting a very long time to find a used unit in as good shape and for that low a price that is 72".