Based on your info the single plane manifold is better while running on nitrous.
A good converter for street/strip N/A and nitrous use is a 10" "3500stall" converter.
What are the inputs (engine specs) you used to come up with those Ddyno results?
Unless you're building a dedicated purpose built nitrous racer build the motor to run good without nitrous.
Adding and additional 100hp using nitrous is easy. But most of the time you'll be running on the motor without nitrous.
A single plane manifold may not be the best choice overall. its depends on your total combination and how you'll be using it.
Ddyno does not accuratly show the power potential of some of the better RPM type dual plane manifolds with a modified plenum divider. These manifolds only give up a few ponies as compared to a single plane manifold on anything shy of a race motor. On a street motor its really a toss up as to which is better.
A 350 with a single plane manifold will want a 3500stall converter minimum and steeper rear gearing.
As a general trend; a single plane manifold has stronger power above 4000rpm than a dual plane but at the cost of torque at 3000 and below.
DDyno shows this.