Good call. Sometimes i impulse buy stuff while stuck at work and forgot the car only had a two bulb taillight. For this reason i think i will return these boxesLike to see how this plays out... with the 1979 tail light housing having only two seperate brake/running lights on the outside with a backup light one in the middle, this wont exactly show sequencial, in a sense as if you had three directly in a row, ie, the newer mustang tail lights.
You could go this route...