Awaycaboose, heres a thought, split a cassette if you can, file the retaining pins at either end off and build a custom five speed in the centre of the freehub with spacesr to either side. You'll have the gears you really need with less danger of the chain being too far out of alighnment. Get hold of some longer rear mech adjuster screws from a hardware store, I think the threading is pretty standard. Adjust the mech over the narrower spread of gears and even fit a thumbshifter, now thats retro !
Alternatively buy a cheap screw on freewheel rear atb wheel and a freewheel plus old/cheap mech. Old school, cheap and works
My money no object plan would be to fit a Shimano Alfine 8 spd hub gear that would work perfectly with a on-one inbred. Genesis and several other firms are now using this idea to get mud friendly mtb's onto the trail.