Headers will provide from 3-15 HP on a modified engine. Headers are difficult to install, produces excessive heat under the hood, and increase noise level compared to factory type exhaust manifolds. The next consideration is larger exhaust pipe if the engine produces 350 HP or more to take advantage of any preceived air flow. I have a 74 Duster with all of the advertised go fast goodies,360 Mopar crate short block, eddy heads, intake and carb. Combined with TTI exhaust system with 2.5" exhaust pipe with X-pipe. 355 gears with 8 3\4 rear end on 15" BFG rubber. A lotsa money to have the 72 Duster with 30 over 340, hughes cam, 323 gears and standard factory stuff. The 340 walks all over the 360 from an idle to top end. Decide what you have and where you want to go, a dual exhaust with 2 1\4 or 2 1\2 pipe would probably provide better performance and economy than a set of headers.
