That flapper valve directs hot exhaust gas against the underside of the intake manifold to help vapourize the fuel when the engine is cold. No problem with a header if you run an electric choke - if you need one!
Most of my carbed cars didn't have the choke hooked up even in our Canadian winter.
Your question and TOHCan's reply got me wondering what that looks like on the six. I didn't find a diagram so I could visualize it but did find this description:
Thanks for the info! I'm assuming, then, that the flapper valve should be in a closed, or semi closed, position when cold and then when the manifold gets hot, the flat coil spring(s) will allow the flapper to go fully open. Does that sound correct? Reason I'm asking is that the springs and flapper are not in great shape. The entire manifold is very pitted. That is also the reason that I'm considering a header. I guess if I can't get the flapper valve working, I can just remove it.
The best thing to do is go for the header. And, since you have a 1-Bbl set-up, you should also switch over to a 4-bbl intake and carb to go with it. With that, a bigger cam would certainly help. Then you should think about slightly higher compression by switching to either a 230 1-Bbl head or the 250 4-Bbl head. Some mild porting and a set of better valves and springs, and you'll be all set!
To remove the flapper you will have to grind away two hot rivets, then cut the sahft adn plug the two holes in the exhaust manifold.
All of that stuff sounds wonderful, but since I just had a valve job done on the 1-BBL head and replaced my cracked intake, I will stick with the 1-BBL setup. Is there a header available for 1-BBL?
Due to the design of the 1 bbl intake with its very short runners it doesn't fit with the header. The Sigma 6 header is designed to work with the stock 4 bbl intake since that combo will have the best power potential. A Stage 1 cam can be added to your present head to complete the package.
Keep the 1 bbl intake and exhaust set in a safe place and bolt on some hp!
The best thing to do is go for the header. And, since you have a 1-Bbl set-up, you should also switch over to a 4-bbl intake and carb to go with it. With that, a bigger cam would certainly help. Then you should think about slightly higher compression by switching to either a 230 1-Bbl head or the 250 4-Bbl head. Some mild porting and a set of better valves and springs, and you'll be all set!...
Q, your putting ideas in my head. I wish my wallet would let me.
I wish my wallet would let me. 
