I think it might be easier to start with tranny, i found out it is R151F, 5 -speed manual, dont know if it is same as in early Turbo Taco. Maybe same development version? Anyways, that might be something worth to investigate... ?
an r151f is a common tranny, i have one, however there arent any lower gears availiable for these, what we do is lower the low range gear set in the transfer case and axle to achieve a lower gear ratio. if you have a 151f then you may have a common transfer case that has parts availiable for it, or you could just buy a gear drive case from marlin built with lower gears as it will bolt up to your 151f easily.
