Are headers worth it on a 91 2500 454?

[cjK3500]

I have broken exhaust manifold bolts on both sides and the manifolds may be cracked (who knows). I'm thinking about getting headers, either Hedman Elite or Thorleys. Is it worth the work? Will it help power and mpg much?
FYI its a 454 with the 4L80e and I'm only getting ~12 MPG. Does that sound low?

Thanks!

[slowpoke]

Here is what i know......very little
With headers you will some sort of starter heat shield, with out it when run the truck awhile, turn it off to run into the store etc., comeback typically it will be hard to turn the motor over or wont at all.Not till it cools off.
Taht experience is based of af a hotrod I had 10 plus years ago.
maybe things have changed.