What overdrive unit for my diesel?

[71-LS6]

After searching for the last month, I've decided to go with a unit called the "Auxillary Transmission" from a company called US Gear. The unit is rated to 35,000 lbs GVWR, and is available for GM, Ford, & my Dodge. They are available with an overdrive ratio of .80 or an underdrive of 1.25. The bad news, is that on my 4WD truck (maybe all trucks, I didn't care about the other applications) they mount between the transfer case and the transmission. Aside from the obvious modifications (driveshafts, linkage, ect. ect.), I will have to remove a little floor pan from my rear passenger area. On a Dodge, that will leave me one seat designated for children and midgets only.

The cost will be somewhere north of $4,000.00, but at 1,000-1,500 miles a week, it should pay itself off with in a year. If this works out, I'll start learning how to burn manure and Cheetos too, gotta do something fast, my fuel bill is eating into my racing budget.