Has anyone entertained the idea of commercial axles? I think some of the Petes and KWs had a variety of ratios to choose from. Keeping both rear axles as drivers is still possible since those axles have an output shaft through the axle housings (for the rear rear) instead of over the top of the differential. A lot of utility company trucks even have live steer axles so they're out there too.
True they aren't the "stock" double reduction type, but if you're suddenly able to get comfortable, non-redline 60 and 70 mph freeway speeds ?!?
Personal experience with 5th over transmissions; I put one in an M-37 and found I'd just passed the h.p./torque threshhold. The truck could barely hold it's own on flat ground, without a headwind, in 5th and was in effect faster in 4th than in top gear.