Hello I am getting my original York heat pump replaced with an Amer. Standard and TAM7 air handler. The York air handler has a side return but the TAM7 will have its return on the bottom so it will be raised off the floor. The HVAC contractor came out but did not take any measurements inside with the duct work. The air handler is in the basement. I am concerned that the supply plenum with be sub par. Should I be concerned or will they fabricate the supply plenum onsite to fit correctly. I have attached some pictures of the current set up. The return is out of the top of the York air handler. The York supply opening is roughly 14x14 and the TAM7 is roughly 14x18. How should it be attached correctly to the duct work?