Sorry about that, my typing overtook my memory recall ability. The close ratio Borg Warner Super T-10 started in 74 and was used in the 74 SD455 cars. The SD455 still had Muncies in 1973 but if I recall correctly, it was the M20 wide ratio. The regular 455 got the close ratio M21, (the M22 got dropped after 1972).
BTW, all the transmissions used the "M" in front of the number regardless of who made them (not just Muncie) and what year.
There's some SD455 trivia on the trans question half way down the page here:
http://www.angelfire.com/mi/jamos1/sdtrivia.html