could the intake be holding my low end down?

This conversation should be limited to stock stroke 360s only because if you include stroked big blocks the needs are completely different. I only use single plane manifolds on my 440s because of the amount of torque that is produced down low, do you need a two plane intake manifold on a motor with 600 ftlb @ 2800 rpm, hell no. Ìts 500 ftlb @ 2500 for a low compression stock stroke 440, single plane on this one also. A two plane can and will limit upper rpm power.