74 Dart. Headers vs manifold.

Anything more than a good tune up your wasting money looking for mileage.
Depends on how much you drive but even to recoupe a $1000 could take 3-5 years. Cheaper just to dump it into the tank.

Now if your looking gor added performance with little to no loss in mpg thats different.

Headers and a good exhaust should add a nice boost in performance across the power band. And set the stage for future up grades 4bbl and small cam etc..