Swapping for an OD 904?

From Allpar:
"Mopar Action’s Rick Ehrenberg wrote that the basic TorqueFlite design continued long past the time when the transmissions were so labelled. In 1989 and 1991, integral overdrives were added, resulting in the A-500 (based on the A-904) and the A-518 (based on the A-727). According to Rick, the engine computer controlled the electric overdrive unit built into the extension housing."

You are probably thinking of the A904 lockup function. Which was implemented around 1977-78. Which basically reduced the cruising RPM by about 200-250. But IMO it made the trans slightly weaker.