Several years ago a buddy of mine who owns a flow bench did a flow comparison between the various exhaust manifolds. The test was done out of curiosity and from a similar thread on moparts posted at the time. The test was to measure flow and port velocity. The manifolds tested were the B body magnum driver side, C Body magnum Driver side and my 68 GTS 383 driver side manifold. All the subjects were tested at 25" of Vacum. All the exhaust ports were tested to come up with an average

Both the B and C manifolds flowed within 5" of each other. Both had the heat riser with the flap wide open. The A Body driver side manifold flowed slightly less on average between the B and C. The number wasn't significant. It had the heat riser removed.

Of course I went looking for the charts in all my stuff that's still unpacked from our move two years ago and have yet to find them. I'll be going up north next month where I can get copies to post.