I Installed a set of Doug's headers on my 340-.030 over 68 block with 71 J heads with 2.02 intake valves.
I noticed a huge improvement from idle through the entire RPM range, and I am only running a 9/16 ths street cam.
I can tell you that headers were one of the biggest performance improvements I've had made!
I plan on installing a Edelbrock RPM intake to replace my cast iron intake , not sure how much I will get with that change?