Ross wrote:
Since the OD is an add on at the back of the normal trans, if you leave the plunger in you will get the overdrive ratio applied to all of your forward gears. First will feel like second, second almost like high, and high will be .72.

A very expensive problem occurs if you try to back up with the OD engaged as the overrunning clutch will go to war with the shifter pawl and one of them will lose. There is an interlock inside the trans to prevent that but in well worn units someone can force it into reverse. When you go to back up chances are the engine will just stall.

Do you still have the overdrive relay hanging somewhere on your firewall? It looks like a voltage regulator but has 6 terminals. It is really not that hard to make the whole thing work and it is very cool, practical and enjoyable when it does.