What to expect out of mild build 360
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.
This E/T calculator seems to check out.
1/4 Mile ET Calculator