I'm replacing the heater core on my '69 right now so I just took the heater box out of the car.
If you unbolt the inner fender well from the car, you can push it down to access the heater box bolts. Once you get the fender well out of the way, the box is pretty easy to get at. The heater box is in two sections. One under the dash and one in the engine bay. It looks like you'll have to take out the heater box section in the engine bay to get at the heater motor.
Once you take the heater box out, you may have to replace the gaskets at the blower motor and where the box attaches to the firewall. National Parts Depot has a gasket set.