I'm looking for front wheel spacers but all I can find are in the neighborhood of $225+, way too much. I can use 1" spacers as all I want them for are clearance for front tire chains. I had bought a set of front chains but the cross chains are of a length that the side chains are right down to the actual rim on both sides. That leaves very little clearance between the chain and the steering knuckle/tie rod end. So close, in fact, that if the chain were to shift some in use I would have a problem. There are inexpensive spacers out there but they have 1/2" thread and the Kubota has 9/16".
I cannot help with the specific question. But since myself and others have found spacers for the rear of a BX cheap on ebay or other online sources, maybe if you look you can find something that will work on the front. Rear spacers in the 1 to 2 inch thickness for the BX were in the $50 range per pair delivered. The spacers I am referring to were actually advertised for use on a jeep, but work perfectly.
Maybe if you take measurements you can find something that works...
OR maybe if you get lucky, someone here will have the answer. That would even be better.
edit: I missed you last sentence for some reason! (red face here)
Actually, I found a set of spacer that might work for my B2910, with the same issue. I was thinking I could drill the holes in the spacers out to 9/16" for mounting to the tractor, but have not figured out what would be necessary to do to adapt the wheels to the 1/2" studs on the spacers, that are used for wheel mounting. Perhaps nothing if the nuts are tapered where they enter the wheels? Not sure about this...