Cheaper for you to go with a C4 if you have a flat head .
I added an OD to my 49 wagon and did not change out the rear axle gears. Added this relay last year. Works as it should. Mounted it to the brace above the left side kick panel. Ran the wires down the kick panel across the floor under the trans cover. Wrapped in Teckflex loom.
https://vintageautogarage.com/borg-warner-r10-r11-overdrive-relay-12-volts/
Adding the kick down switch was a hassle for some reason. When I did mine, took the time to replace the motor mounts. Be sure the rear bushing and seal are replaced.
Used the same drive shaft. Check your axle seals for leaks. While you are wrenching in those areas, easy to replace.
If you are willing to forgo some of the automatic features of the factory control system you can use much fewer parts to install the overdrive. The minimum you need for easy install is the transmission, a good solenoid, and the lock out cable. An on-off switch to run the solenoid takes care of shifting in and out of overdrive. For anyone going to Auburn I can demonstrate this in my 53 Ford.
The 49-early 51 Mercury transmission looks like this:
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