You may wish to
check out my thread. I used one of the new 28h SRAM Automatix hubs.
Needs a few mods i.e. modify the change up speed and take off a locking nut/mill one down.
You need a dished rear sprocket to get the chain line right. The smallest dished one you can get is 16T and with a 54T main, it will give you 56" and 76".
I modified mine to auto change up at 11 mph.
Regards
Jerry