Red plastic wheel house liners were a $46.55 factory option on 68 Barracudas (fastbacks and hardtops only). And I have the original window sticker showing the car was ordered with them. This was the first Barracuda I had ever seen with that rare option and that was the main reason I bought it back in 1985. I have since seen a few others with the liners. Since the shape of the inner rear wheel wells was a bit different on the convertibles, you couldn't get the liners on the convertibles.
A similar option was offered on 1966 Pontiac GTOs, but on the 4-4-2's you mention, the front fenderwells are actually painted red, they aren't plastic liners like on the GTO and Barracuda.
The plastic liners were an appearance only option not intended to have any practical use. But, they did prevent rust in the front fenders and if the trailing edge of the rear liners had come down a bit further than they would heve protected the lower rear quarters as well.
