If you run it off the gear box you'll want a 1:1 ratio from the engine to keep the clutch in the correct operating rpm for our hp level. I don't think that would be possible to do or your axle sprocket would be bigger then the rear tires. Anything with any power at all won't lock up tight enough if the clutch hub will only see 4000rpm or so. If you're going to put three times the power through the clutch than it was designed for, you'll have to spin it faster to keep it locked up the best you can unless you go to a disc clutch where it can be made to work at any rpm.