We built a 4 wheel pivot for last year. You can get to all of our drawings and calculations here
DEWBOT VI Drive Train. One of our mentors (GDeaver) figured out a easy way to mount the tread on the wheel, you might want to look at that. (His posts are linked in the main article)
Our pivot has more material than yours for support. It's been through 2 regionals and 4 off season and is still in good shape. You might want to add some more support.
Good luck!