355's to 391's how much does it help?

My daily is stock 73 340 dart sport, 3.23 2800 rpm 70 mph 1/4 14s spinning, fun car and drive it anywhere. 3.91s in my other dart, local driver and drags TNT, runs 12.5 and a blast to drive!
Pick your rear gear and tire diameter to match your cams upper power band crossing the stripe.