Anyone install three speed with overdrive behind a slant six

All right are you talking about the 4 speed modified to have an OD gear, or are you taking about an acutal 3 speed with an OD unit? How many shift levers does the gearbox have?

This is a Mopar 4 speed overdrive Notice the 3 shift levers and notice the front lever POINTS DOWN. This was originally a 4 speed design, and Chrysler re-worked what had been 3rd gear into an OD gear. Then they pointed the 3-4 shift lever DOWN which reverses it's action. So when you shift the OD box, shifter 1-2-3-4, the box itself goes 1-2-the old 4-the old 3--which is the OD gear

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Below is a generic photo (probably GM) of a 3 speed overdrive gearbox. NOTE that it only has 2 shift levers, R-1 and 2-3. The round electrical unit shifts the OD in/ out

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