my 66 40 came with a new overdrive in the box. I don't drive the 40 much and was wondering if it would be worth keeping and adding it onto the trans or just selling it.
The 40 is bone stock with some 30 inch mud terrains only
IIRC those go between the clutch and tranny, if you were dropping in a V8 that would move the engine forward and adapt the tranny. Sell it is my opinion.
The Ranger went through a couple (at least) evolutions. Even the most recent is now obsolete and unsupported. They are stout, well designed units, so this by no means makes this a non starter for a conversion.
To the best of my knowledge the ranger units worked behind a Chevy or a Ford bell housing and that was it. I believe it only works with the 4/5 speed tranny as well.
In terms of the sell or keep question. Unless you want to do an engine and tranny swap/conversion... sell it.
If you only drive the rig a little bit and do not routinely push the limits of what you have... sell it.