I talked to the "tech" guy at Hooker Headers when I bought my headers for my '69-351w. He told me that ( as far as horse power increases, etc.), the shorty style headers are about half way between stock exhaust manifolds and full length headers. They do make more power than stock but not as much as full length headers. I figured that if I was doing the job anyway, might as well go all the way! There was ample room, Even though the job was a pain, it was worth it. Hope this helps.