I'm certainly no professional wrench, like many on this and other boards, so when I figgerd out who was...I followed their advice. Primarily PinHead and Poser (I know ther are others). There are a lot of variables involved with many of the "improvements", "mods" and "upgrades". The results can be deceiving.
When you replace a stock manifold, intake, exhaust, headers, makes no difference, you replace the gasket on the manifold(s). IF things were not right BEFORE you replaced anything, and they're "right" after, then it is easy to give credit to whatever you bolted on. When in reality, you just may have installed everything CORRECTLY!
I have a '74 F motor in a '66 FJ40. Replaced the intake/exhaust manifold gasket, had only RTV before, well it improved, but still a rough idle. Rebuilt the carb...sorta, and performance improved, but still a rough idle. Added Mini truck PS, and wanted to retain a stock aircleaner, which meant a 2F, because of the location of the PS pump. Decided to run a 2F valve cover and intake, which would not mate with my F exhaust manifold, so I decided to go with a header. This time I really rebuilt my '74 carb, boiled out, etc. Used 2 gaskets on the header, fired it up and VOILA! Runs great and idles smooth at 550 rpm. Everything I ever wanted!
Now was it the header? The carb rebuild? The HEI? The PS?
OR...was it the fact that everything was installed correctly or at least better than it was before? I think I cured an intake leak that was never properly identified before. These rigs are OLD, many unprofessionally wrenched. The amazing thing is it still ran, as most of them do.
So with all that in mind...HEI, Header, carb rebuild, PS...oh, and a new valve cover, polished, of course.

Ed