Don't believe the engine size or transmission type effected the type of solenoid your car came equipped for
For a April built NJ Mustangs I would think that the style marked 2701966 and FoMoCo (in the block style) on the top, black Bakelite body, fine thread main connections and nuts with toothed washers would be the best choice. I do have some examples of the C6 version but later in production and different plant
Unlike the style with the vent in the bottom metal plate an original won't be dated from the ones I've seen. There are reproductions out there or if you find/have an original those can be rebuilt from my understanding
Did find a different one on a mid April dated NJ example that was pretty original but on closer look it was a later 67 or D2 version based on other characteristics IMHO
Hope this helps
Sorry due to the lighting at the time the picture was taken it has a yellow cast in the picture below.