I don't have any experience with the STI BLack Diamond but I was seriously considering putting a 24x11-10 on the modified 10 inch rim.
The specs on the bottom of the page list that tire as being 24 pounds. A 25 inch one is 25.4 pounds. The tire I chose to go with is 16 pounds. Many of the 8 ply UTV tires are over 30 pounds. I think that the Yamaha engineers did an admirable job of making the BW overweight. They don't need any more help from me.
I only have a bushing for a 350 available and that has a different part number from the 200 bushing. The thickness of the flange is 3mm. The length of the the bushing is 45.5mm. O.D. is 32.8mm. I.D. is 26.8mm.
The eccentric bushings are 65mm and 83mm long. In other words, the eccentrics are a lot longer than the plastic bushings. There is a lot of the bushing that is unsupported in the bore of the swing arm. I suspect that means that the actual length of the plastic bushing isn't that critical.
I still haven't been able to see any of your photos yet except your diagram of the bushing.