Otto - Your recirculating heater and core are different than Johns earlier style heater. Apples and oranges here, so they aren't the same to begin with and his core clearly has a smaller diameter motor recess.
John - I think you will be fine on the contact between the motor case and the core honestly. It's not like it's going to act like a huge heat sink for the core or anything and since it designed to be in an enclosed case, it's meant to be able to handle the heat IMHO. If the motor wasn't up to handling the heat, it would have been designed with air flow openings for and aft on the case to allow for cooling air flow.
