I'm thinking about trading out my light duty 3 speed tranny for the four speed on my AA '29 Truck. I would like to keep my
high-low two speed. Was this an option at one time or If this possible, what parts and pieces would I need to do this conversion and hook the two together?
To replace the 3-speed transmission with a 4-speed and retain the dual high requires an AA-7085-B rear bearing retainer and a special u-joint to fit between the two transmissions. Your AA has the dual high rear support currently.
I should have the AA-7085-B retainer to sell. I don't have the u-joint.
The Ford Garage link above shows the AA-7085-B retainer. And, I believe that the u-joint front knuckle shown would be needed along with the rear knuckle of your current u-joint.