From my experience, lowering the divider loses torque significantly, especially 1 1/4"s in a 289 manifold. On some manifolds, milling the divider down like that will cause a stumble below 3000 rpm, due to fuel coming out of suspension. If it was milled level, front to rear, then I personally would suggest welding in a divider and restoring the separation of planes. Use a spacer if needed afterwards. Joe-JDC