Chaz 58,
The "bolt" next to where the cable housing screws in should be retaining a metal tab that keeps the speedo drive "plug" in proper position in your tailshaft housing.
Without knowing exactly which "3 speed overdrive" you used as a replacement, it's hard to determine if you overfilled it or not.
Thinking you need to do a bit to determine what you have, check appropriate shop manuals for type/vintage and check back in with updated info and question.
A picture or two wouldn't hurt either.
