On multiple cylinder engines, a dedicated exhausttube like in a header will scavenge a cylinder better than a combined log-type manifold will.
You still think in terms of flow, while you should be thinking in terms of 4 (hopefully) equal tubes ending all together in a plane in a formed collector where each tube is 'helping' the other tubes to scavenge/pull out the exhaust away from the cylinders.

Besides all this, my believe was that only A-body Mopars have space issues when it comes to 3gen Hemi exhausts.
I also have 5.7Hemi Dart in the works and I'm currently still convinced I will be making my own headers for this setup.