Hello, I am installing a new furnce 96000 BTU. I need to upgrade some of the ductwork. I ordered a supply plenum that is 19 x 19 x34inches tall.(which is the size of the furnace outlet) My blower speed is 1570. Is this plenum too large? when I plug this into the ductulator my FPM is only about 600 FPM. I am not sure if I need to transition this plenum to the proper CFM closer too the blower or if the transition is fine at the take offs?