Mine is a 1965 Plymouth Barracuda & I am unaware of the dispersement of the 273's!
For my Barracuda, it shows that the 1964 production year produced: A Straight 6, A Slant 6 & A 273V8 were the only options for the 1965 Model Year! I am not aware of a 4 Barrel Factory Model Year Option Available for my car.