I did this on my 68 and I am assuming your 70 is basically the same.
The duct will need to be removed. The duct is like a clam shell with the largest part inside the fire wall and the other smaller duct in the engine compartment that houses the heater fan. The inner duct has studs that penetrate through the fire wall that fasten with “speed” nuts. Bad news is, you may have to remove or loosen the right fender to get at all the fasteners.
The insulation is held in place with ribbed push-in plugs like the ones that hold the inner door panels on, just bigger. To get the old one out, use wire cutters to snip them off from the engine side then use a punch to drive them through. Putting the new one in is awkward. Get a helper to hold it in place while you push in the new plastic plugs. A little spray contact cement may be required to secure some areas of the new insulation to the fire wall.
Consider removing the front seat(s) for easier access to under the dash. And if you have any suspicion that the heater core could fail in the near future, now is a good time to replace it.