I'm with Glenn, before ordering parts for my stroker build I read a "how to build high performance engines" book. It states,
that if an engine is tuned properly, it will always make more hp. But having back pressure can help tune the torque curve, so you might opt for manifolds in a motorhome engine over headers, get it? So for this reason I vote B.
The only exception I would say, is that heads seem to flow slightly better with short primary tubes attached. Then again, I wouldn't consider a 4" primary tube a backpressure creating restriction. It probably just acts like an extension of the port and somehow smooths the air flow to increase velocity?
So once again, talking horsepower, and not developing torque at a certain rpm, the engine will always make more hp with less back pressure. Assuming of course the engine will be tuned to run correctly in that state.