^^^^^^^^^^ What these Guys said..^^^^^^^^^^^^^ Your Trucks being like it for over a quarter of a century, Just relax & appreciate it for what It is...... A Classic Old Truck that's Still working... Just my 2 Cents Worth..
Have given any thought to a 15 spd OD? Gives you 10 road gears with a .78 OD gear and the option of deep reduction if you ever need it.
Why do you think you need to go to an OD when the OD is less efficient than say changing the axle ratio?
That's what I thought. I've spent lots of time with people that are well educated on the subject and they all have said the closer you get to a 1:1 final drive ratio, the more efficient it is.
!0 speeds are everywhere and cheap, hell I have one I have no use for, but shipping would be a bear and I have never checked to see if it is overdrive or not. probably cheaper tan finding and swapping reads. everyhing is a trade off too lower rear gears means harder to start a load off from a stop and 264 gears would render a truck useless for anything I would be doing, but may well work for him..