The 4th gear ratio in the 4-speed trans. is 1:1. The 3-speed 3rd gear ratio is 1:1. There would be no high end advantage. You would have to double clutch with the non-synchronized 4 speed, not the 3 speed. The floorboard may be different too. First gear in the 4-speed isn't referred to as a granny gear for nothing. You could walk faster. If you decide to go the 4-speed route, I'd be interested in your 3-speed trans.
Rick
