you might want to try a"RAM"or a "ilton"multiple plate clutch assmbly.the diameters of these units are around 6 inches and have a low rotating mass,they also use a flexplate rather than a flywheel (it's used for the ring gear only).the clutch assembly bolts directly to the crank shaft flange.there are a number of vintage racers that use this setup.if you go to
www.ramclutches.com and look up their "assault weapon" system .