GM did not sell an electric clock for the 54/55 trucks. A manual-wind clock (986633) was sold as a truck accessory.

Manual-wind (986633) and electric (986971) clocks were sold for cars. The car wind-up clock fits the housing used in a 54/55 truck.

Maybe someone knows if the car electric clock used a different housing (or escutcheon/trim-ting) in a car (than was used with the wind-up clock in a car)?

manual-wind clock dimensions:
3 and 1/8" outer diameter (outer edge of trim ring)
2 and 3/4" inner diameter (what the inner body/barrel has to clear)