I built a low budget version of this on our 23 T-bucket with a Turbo 400 trans.
I used a 4" linear actuator from
www.uslinear.com mounted on the trans with a pivot on each end to actuate the shifter linkage.
I ran that through a power window switch. I ran the power for the switch through the brake pedal so the ONLY way the actuator can get power is if the brake is depressed. Therefore the only way it can go in and out of gear is if you are braking/stopped.
On the t-bucket you can feel when it goes in gear so there is no need for a indicator switch.