I have a moderately modified 340 (+/- 375 HP), and I really like my Air Gap. In your case, I think it mostly matters on what your plans are. A good dual plane like the Air gap will give decent low end torque and decent higher RPM HP. Whereas a good single plane will be better at higher RPMs and you might lose some lower RPM grunt.
I am going to quote myself here. After reading the Op's thread a little more carefully, I am going to go with the Airgap. It's an awesome manifold, and if he wanted more HP in a higher RMP band, a manifold swap isn't that much trouble.