Dragula...that big 426-4 manifold you have on yours really needs to be wound up to run naturally aspirated. A fairly tight converter and no gear will kill it. The Ray Barton single 4500 manifold makes a lot more lowend grunt than the -4 does. The dual plane looks nice, but I bet it'll neuter your big 600" motor enough that it won't go 9s anymore even though it would definitely 60' better.
I tested the 426-3, 426-4, and the Barton manifold on my hemi and got a decent handle on how they all "act". No matter how they look, there's a pretty big difference in all 3 of them. The dual plane would be a good bit below the 426-3 in my opinion. All lowend and nothing upstairs on anything fairly sporty.