I've never looked that close to see if it could be done, but.
Would it be possible to use the stock speedo trans gear/cable assy, and put the ratio adapter on the other end of the cable, then have a short cable to connect the ratio adapter to the speedo head? Similar to the way mopar did the cruise control.

Charrlie_S
66 Cuda 170 nitrous
66 Valiant 225 nitrous
65 Valiant 170 turbo
64 Valiant driver
64 Valiant roller