Thanks Dusty and Rust Addict for the help on my seat brace questions. I'm beginning to think Kubota doesn't have a very good grip on configuration management! The page out of the parts break down that Rust Addict shows looks very much like my tractor, with the item 10 rubber block. My dealer said my tractor must have had a "backhoe kit" on it at one time, which was what the #10 rubber block went with. However, the Kubota B6000 Illustrated Parts List I bought from my dealer, and the digital parts breakdown he uses, show a seat brace like your picture shows! By the way, the seat in the picture looks just like the one for a B6100 my dealer showed me. The seat for mine only has single "rails" (on the outside) that are pinned to the rubber bushings, where the B6100 seat he showed me had inside and outside "rails". My dealer said the seat brace is supposed to extend to the fenders (to keep them from "flapping"). My plan is to make a seat brace like that shown in your picture. I've already purchased the "rubber bumpers" and although my fenders currently have no holes to bolt the brace to, I'll just drill some. It seems like it will make a much sturdier set up.
Thanks again for the help.