Agreed. In fact a single spindle cutter is designed to go between the wheel tracks for an optimum cut. I'm not sure where the whole notion of having a cutter wider than your tractor came from, but as per the design of the cutter that is incorrect. The reason for this is so the cutter is cutting undisturbed grass. With this setup if you go around the field the proper way the inside tire will still mash down the grass for the next pass, but the rotation of the blades will pick it back up, the outside tire is always in cut grass.
I use a 5' bush hog on my 5203. The cutter works better on that tractor than my Ford NAA or B3200. The distance between my front tires is right at 5', so it's real easy to judge where you are cutting and get a full swath each time with very little overlap. If I need to cut up close to something I just swing it over to one side using the sway links.
So I say go with the 6', a single spindle 7' is a really large, heavy and long cutter. They also take a disproportionate amount of power due to the really long blades.
This is an illustration provided in the manuals for JD single spindle cutters describing cutting technique and wheel spacing:
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