340hp sparkplug choice

the reason im asking i ran down the strip and it didnt do very good. it broke up at high rpm. 2.2 60 ft 91mph 15.2. 1/4. My stock 273 did 14.7 in the late 60s.needles to say i wasent happy so its time to go over the details. Its close to stock, hp. manifolds. i think it should do a lot better than that