I don't understand how/why you can't use a manual shift auxiliary. If the lever does not come out where you want it fabricate linkage under the truck so it will come out in a more acceptable place. Or switch to heavy duty push pull cable https://www.midwestcontrol.com/part_family.php?id=3

I suppose you could make an air shifted version but I doubt it would shift as smoothly and kindly (for the transmission) as a lever. Here are some other peoples thoughts about the same.
http://www.pirate4x4.com/forum/general-4x4-discussion/619676-ranger-overdrive.html

There have been electric shift shift versions by other manufacturers
http://www.ford-trucks.com/forums/917000-auxillary-transmission-between-bellhousing-and-trans.html

Here is a current electric OD that goes after the transmission I hear mixed reviews for these and the price is quite steep.
https://www.gearvendors.com/



•1951 GMC 250 in the Project Journals
•1948 Chevrolet 6400 - Detroit Diesel 4-53T - Roadranger 10 speed overdrive - 4 wheel disc brakes
•1952 Chevrolet 3800 pickup
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"First, get a clear notion of what you desire to accomplish, and then in all probability you will succeed in doing it..." -Henry Maudslay-