Shorty Headers
in a mild application shorties offer nothing when compared to 340 manifolds. long tubes with collectors will pick up hp & torque across the band and while the gain is not tremendous that little bump of 10% or so can convert into drivability and throttle response.
with more aggressive cam, intake and heads the gains increase. both Dougs & TTI are about as good as you can get fitment wise without going to a full custom header.
I had 340 manifolds on my 64 & 65 darts (360 w/ magnum heads & M1) and switched to shorty headers and there was no applicable difference (rwhp/tq remained nearly the same). I then upgraded to Dougs in the 65 and immediately noticed the difference in the "butt dyno" and overall performance of the car.