Are you asking what direction to put them or if you should use them at all.
If I you were to use them, putting horizontal would be best I guess.
On using them, NO, NO they are ugly on a Mustang, on a Model A sedan they are hideous!
There are all kinds of cool lights with LED inserts available that are actually brighter than those, MUCH brighter. Add a third brake light and you are cruising safely.