Again, Robert, putting an overdrive box behind the tranny is not so much a matter of cutting down the driveshaft, as it is moving the whole transfer case back a foot or so to fit such a device in there.
See, the tranny and transfer case are pinned together by a tiny little "spud shaft", which is a driveshaft made from two u-joints with a slip spline between em. Once you put that spudshaft into the equation, there is less than an inch to do ANYTHING between tranny and the accompanying divorced transfer case.
Me, I went to a .695 tranny, with 45" tires. My "theoretical" top speed on the flats is around 80 MPH.
PS, final bit of info, rear axle spacing is at 48".