If it is quarter mile performance there is more to going fast than headers or manifolds. As for Exhaust Manifolds and they are a nice factory piece I would leave them alone and find ones that are already altered or buy a set of repros and port them with no hear riser. At one time in I isle at Carlisle Mario sold Auminum Hemi exhaust manfolds. You save a lot of weight running those and I believe a few FAST hemi cars have them on. Every area of the motor can be altered for more HP like the stock intake manifold with a Vanke Mod,, changing rocker ratios. Leaving a little on the table is not a bad thing.