How fast have you gone with exhaust manifolds?
Here is a website about taking several different sets of exhaust manifolds and headers and comparing them on a mule engine on a dyno. I think the engine was a 360 crate; not sure which one, but the article says, I believe. I just don't have time to look it up right now.
This article is pretty eye-opening, I think.
Click on the little tan-colored menus with the horsepower and torque numbers and they will expand to much larger menus that are a lot easier to read.
Just go to:
http://tinyurl.com/6dwlmv
Pretty educational I thought...
Bill