Asbestos as a board is a lousy insulator for a ducted system and asbestos board insulation for heating equipment ended with the introduction of ductwork in the 50's. I have serious doubts that you actually DO have asbestos insulation in a 70's house. Asbestos lagging was for industrial boiler flue insulation; too expensive and wierd for home use. Have a sample checked to be sure. Then use the person who provides the truth.