I think Death Racers are running 26's(or smaller) and pedaling like crazy to get back up to speed.
heyscuba, running a J-shaft on a single speed would be a waste of money, you'd want to have at least a 3-speed hub(low-med-high/hill-cruise-flat), but alot of those can't handle the torque. You can get 6-8 speed hubs to make you more efficient, or even use a NuVinci infinitely variable hub if you have the $.
But there is a whole section here just on hubs, the frequenters there will be able to give you more solid answers than us General guys
Motorized Bicycle: Engine Kit Forum