Header recommendation, 1968 Dodge Dart, 340 4speed

i did read dyno tests on a 300 hp 360 done by Mopar Muscle.Headers averaged about 12 more hp than the hi flo manifolds.Oddly enough the Hedman Shorties outperformed the Hooker Super comp and the TTI's.
Dyno Tests were with open headers ...might not be much gain through another
12 feet of exhaust pipe and mufflers.