Depends on what you plan to do with the rig I suppose.  Are you trying to set some land speed records, creep through the rocks at a fraction of a mph, or something in between?  It's also going to depend on whether you're planning on running a manual or auto tranny as to what the appropriate gearing might be.  By the way, what is the engine going into and do you have any plans to go dual cases?  Give us some more details...and pics if you got 'em because this sounds like it could evolve into a cool build thread.