Sizes of Thumb Wheel Radios
Here's some pictures from a '68 Barracuda. I believe they are different than Darts and use a different bezel on the front.
Instrument panel bezel is removed. The radio mounts from behind the dash. The opening in the dash only needs to be big enough for the wheels and dial to fit through.
Dimensions of dash opening:
Different radio with chrome bezel. Note holes for mounting bolts on upper left and right side.
Exterior dimensions of radio: