Hi Steve,
I recently installed a RAWLES manifold and there is a noticeable difference in performance across the rev range but I think
to get the full effect you need the proper camshaft and that might be a good topic for discussion.
I have a Denis Welch DWR 8 cam (rated 3-5,000 for fast street/rally) in my engine and a set of his headers but the engine seems to run out of steam a little before 5,000 rpm.
With a BJ8 cam and the old intake manifold I was able to pull 6,000 rpm if needed without damage to the engine.
For better performance I'm gonna put the OEM BJ8 cam back in which is the cam the factory suggest for racing applications!
I'm still surprised the engine ever ran properly with the OEM intake manifold.
View attachment 43051
Nothing ventured nothing gained.
Good luck with your project.
Peter