A998 Trans in '66 Dart

I'm sure one of the "know it alls" will chime in. I believe the overdrive unit is in the tail housing at the rear of the transmission and is larger in that area. So ,from what I understand,the tunnel would have to be modded. . . . correction, I believe the 518 is the overdrive trans and the 998 and 999 are just 904's that could have a lockup converter. this is JMO