I've been thinking about this for a while as to whether or not headers would make much of a difference fuel wise...
​​​​​its 5.8 2wd. I bought this trk. With a hole in #5 cyl. Did not mind cause I love doin this kind of work when I'm able.(screwed up back that can't be fixed). Anyway, its a good truck and I hope its my last one....just lookin for ways to save...thanks

 

Engine go back together stock? Any performance add ons, hardware, software-tuner etc?

What about the rest of the exhaust, any changes being made?
Anything that can free up or make the engine breath better may make some small improvement. Tuner may add a couple ponies and help with MPG, although that always seems debatable but generally any HP gains aren't at any fuel number loss.
Headers alone, doubt you'd see any mileage gain.
The problem I've seemed to have was keeping them sealed at the head flanges and gasket leaks.
One of my older trucks that I ran them on, the exhaust note changed with headers and turbo muffs, and coupled with a slight performance cam seemed to make it a bit more snappy.

 

No....no performance mods although I was thinking of a K&N filter to hook straight to the throttle body in hopes of also letting it breath a bit more...don't want to put too much into this when i have my hands happily full w my 69 Charger...but thanks again for answering, it helped.