Since there's no 6 speed tranny, how about a lower gear set for the first three gears, kinda like the trails set with the lower 1st and 2nd,for the four speed trannys, so we could lower the first three, then put taller final gearing on it, which would put the first three right back where they were, and have a taller fourth and fifth for the highway.
Or maybe different primary gears to make the overall ratio overall taller that way instead of with the final drive?