rclooney,
I assume you are designing the hovercraft. You might consider contacting Fox manufacturing Co. (the makers of Fox engines). Many years ago Duke fox made some hovercraft to play with, all powered by Fox engines. One he rode around his shop. They may have some information they can send you. I think a car radiator fan is a bit heavy. You might want to consider something like a wooden hub and thin plywood blades.
Keep us posted on your progress. The project sounds interesting. Good luck with it.

George