I really don't have any experience yet making modifications to Plymouth / Dodge flatheads, so I can't make any claims as to the performance increase on those specific motors. I do have some experience playing with GM V8's.
On those motors, I've seen a noticeable difference in modifying the exhaust systems.
Just placing something on a motor that is referred to as a "header" is not necessarily going to make it perform better. There are all types of variables that come into play.
One of the biggest issues I noticed in V8 exhaust systems that I've played with was the header length. I've run full length headers on some motors (primary tubes approx 36" long) and the short "block hugger" type headers on other motors.
In fact, one time, I swapped from full length headers on my 65 chevy truck (350 small block) to block huggers and instantly noticed a difference.
First of all, the sound from the block huggers was awful. It actually sounded somewhat muffled, like the exhaust was a little restricted. The full length headers had a much better tone. The power from the full length headers seemed better as well. However, it wasn't a huge difference, as this motor was otherwise stock with the exception of a better carb and alum intake.
Other issues that will affect how your exhaust performs is the manner in which the exhaust tubing is bent and routed, the mufflers, crossover pipes, etc.
As far as the cast iron headers that are available for the flathead 6's, I doubt that there is a big difference in power or economy, but I don't know for sure, because I've never used them. However, they definetely look nice.
My only point in my ramblings here is that there are a number of issues that will affect how well your exhaust works.
