Without a dog in this fight, I think that a very long and complete case has been made for never buying cast iron centers. Ordinary steel wheels have none of these oddball problems and restrictions. Fill the rear tires with liquid, use plain steel wheels, and forget all this. Isn't Kubota about the only one with this crazy circumstance? I do not recall New Holland, Deere or Massey having cast iron centers for weight (?)
Oh by the way, this is one more nail in the Kubota coffin illustrating that the Japanese, Kubota in particular, simply do not understand that people who operate on steep ground need to spread the wheels wider than Kubota allows. Worse on their smaller machines than this M9040 example.
The overszd manual page above agrees exactly with what I found on the dealer lots (using tape measures and calculations) when shopping for a tractor last round. The M9540 widest stance is 75.6" Add the tire width using 18" tires and you get ~ 84 inch best case width between outside edges. I run my MF2660 wheels such that the outside edges of the tires are 96" apart all the time, which I find nearly ideal. There is about another 12" spread available on the Massey with various wheel configurations that I have not used because of trailering limitations, etc. But the flexibility is there.