The gains was a combination of the cams manifold and headers. I think the manifold is responsible for about half the gains. You can easaly see that the 328 manifold is restricted by just vissualy comparing the two manifold runners. I enjoy doing DIY mods so yes for me its worth the efford. I had the manifold so it cost me the adapter plate only.
Testing the pressure in the manifold will tell you if there is any restriction in the inlet, but wont tell you anything about the manifold itself.
I will send the Autocad drawing to you later.