I take you are wanting the .82 overdrive, which is the 600 RR (Road Race) of the TKO family of transmissions. It can be done, but you may be money ahead to sell the transmission and purchase the road race version. Have you called to talk with Shafi or Richard on this. They may still be selling service parts for the TKO and could get you the required gearset(s). If they can't help you, I'd be glad to point to couple of really good sources for the parts and/or rebuilding of your TKO. Let me know.
Tony Edler
Hurst Drive Train Conversions (Classic Mopar 5 Speed)