Just finished a restore on a 71 B Body. The blower motor has a round opening in the side for a soft plastic vent tube to go from it to a hole in the firewall which leads back into the heater box. The front of the motor has 2 studs which line up to holes on a soft rubber seal on a rear metal plate which is secured to the box via sheet metal screws. There is a fibre paper gasket between the front of the motor and the soft rubber seal. Not sure if the A bodies used the metal blower plate or not.