Can these be cut down to 6" tall in order that they can make up the height of the furnace to fit to existing plenums (as new furnaces are 6" shorter in height)?

We do not recommend cutting the box down to 6" tall. The box is 18" tall with the few upper inches being where the internal filter rack is located. Cutting the box down too short will not leave enough room to connect any return duct to the sides.

If you have any further questions, please call us and we will be happy to assist you.