I must of read the same article, because I read the same thing, years ago. I never bought a new 340 car, so I'm not sure if that is correct. All the 340's I have played with had a gasket of some sort. My thinking here is, if I recall correctly, you say you rebuilt a 318. It appears you may have 340 exhaust manifolds on it. The manifolds ports are larger than the 318 heads, so maybe you are not getting a good seat. Another view is, did you use a 340 head ( X,J,O, or U) on a 318 exhaust manifold? If so you will have a opening on the bottom of the manifold and exhaust will escape there. You will need a 340, 360 or headers, if so. I believe the 360 head will be the same issue.