Wow, neat! It's good to know that you have used it for that long without any trouble, especially with that style of axle configuration.
How did you attach the outer wheels?
I built a set of spools for my Yanmar YM2000, a comparable machine to your L185 of about the same size but with a bit more power. I used a leftover set of tires that are a bit smaller (8.3x24 rather than the 9.5x24) but in soft ground, where they are needed, they bit just fine, and added a significant amount of weight when filled:
I ended up taking them off, since I found the width too cumbersome for my orchard, and added 360 lbs of wheel weights instead. The Yanmar was originally designed for a set of duals on it, or a weird paddle wheel arrangement for use in the rice paddies.
I bet your tractor is tremendously stable with the additional width, mass, and traction.