I know a guy up here in Hickory who runs dual quads on top of a tunnel ram on a 318 in his 71 Demon. Never heard it run but it's a semi daily driver so it must run halfway decent.
Mid 13s are plausible if the car is ~3400 with driver. 13.6 would require about 310 crank horsepower. That may be a bit on the light side especially being a later Valiant.
That assumes that the car is driven well in good conditions on sticky tires.