If you need to pull the motor/trans again I would be done with the OD deal. Put a 727 in it and don't sweat the RPM.

My junk knocks down 13-14MPG spinning 3500RPM on drag week for 4 years now. Its quieter and feels more "happy" then it does at lower RPM. If I put an overdrive in I would just have to drive faster lol(not that it would be a bad thing).

68 Barracuda Formula S 340