Someone may be able to confirm this, but I think there are some 5 speeds around from some 50-60's big bolts with Overdrive in 5th gear. Seems to me that would be the easiest way to get some more speed. I know I saw an old road tractor with a 5 speed for sale claiming that the 5 speed was an OD and I know where a transmission out of a firetruck is that is the same model number....so I've been eyeballing it for the same reason.
