Finding the right spark plug for your 340 or 440 HP ..

I don't know where you got this info from, but, for example, the Autolite #86 is the plug recommended for 69 Corvette with SHP 350. Maybe the application is the same for 318 engine, but that doesn't mean it's a "low performance" plug.
Best of Luck to you if you decide to spend unnecessary funds on a "drag race" spark plug for your car.

George