IF the 70 is set up like the 65... I would blame the door that opens the heat ducts/closes off the main vents when vacuum is applied. The vacuum switch controlling the door is near the top of the firewall. Sometimes, when the switch goes bad and vacuum won't retract the arm, folks will hardwire the door open for the winter. If you don't see anything like that my guess is the door has come off its pivot or is rusted in the heat position.
You might try unhooking the vacuum switch arm and manually opening and closing the door, maybe getting a little oil down there.