I don't know about that. Spitfires made a huge difference over 340 manifolds in djvcuda's 66 cuda. They may not give max power but sure an improvement over manifolds in my experience with them. .
I can personally vouch for these headers being a substantial gain over the early cast 273 manifolds, with no other changes. More torque, top end, everywhere up to 6750 rpm. And you won't burn your knuckles changing plugs either. Great plug access! You will NOT accidentally dent these, the tubes are thick. $220 shipped c.o.d. Harold was a gentleman in my case, two weeks shipping. This was a while back...