What's that with stock pipes. 2.5% don't sound like much. Headers will give you around 10% more HP, but that is also a larger pipe to correspond with the header tubes and chamber. I installed 318 manifolds on my 340 when I first installed it in my van. I was not impressed with the motor change at first. Next week I installed headers, not 340 manifolds, What a big difference. I just read your comment again and maybe on a 318, but not a 340, sorry for the add-on.