Hoping someone can help. Current furnace is sitting on top of 17.5 in box plenum. The furnace is old and undersized. The a/c coil is newer and is 21 inch. Looking to purchase 21 in furnace. Newer furnace is shorter by about 6 in. Can I use the existing 17.5 in box stand and build a transition which would be 17.5 in on the bottom, 21 inch on top to accommodate new furnace? Thank you.