Barracuda/Valiant motor question.

The biggest baddest motor you could get in a 65 Cuda was a 273 yet in 1968 my Mom bought a 65 Valiant with a 318 4 brl and it was a pretty quick little car for a stocker. The kid she sold it to took me for a ride in it before he wrapped it around a pole a couple weeks later and that little car smoked the tire/s for a long ways before it hooked. Any reason they didn't offer the 318 in an early Cuda, yet they did on the Valiant or am I just mistaken on this?