Well, while you didn't ask us to "tear it apart" in this picture, I'll do it anyways.
We found that the VEX chain stretches quite a bit over time. I would recommend putting the chain on the end of your shafts for two reasons.
1) A slack chain will probably run against the balls. Not sure if that'll affect the operation of your hopper.
2) Having the tension in the middle of your shaft will cause the shaft to bend a little easier. We had a hard time keeping our spans narrow (especially with the size of the softballs.) We've noticed that the shafts are bending quite a bit, and aren't really straight anymore. So much for 1/8" steel shaft...
It looks good overall. If you can make it two balls wide, I think you'd be better off. Our base is similarly sized, as 12" wide by 15" long. It turns just fine with 4wd. However, with the size of the big ball, you have to watch out for being tipped from the side by it. We almost took a couple tumbles in some pushing matches to get the atlas ball...
Good luck!
BEN