I have a61 Bird that has a blown water heater. How do I go about replacing it, short of cutting through the firewall?

Not sure if the '61 is like the '66 but if it is, you have to remove the heater box from inside the car. The heater core is in the box and behind an access plate but the access plate faces the inside firewall (??!?!?). Time consuming but not a difficult job. If this is what you have to do, remove the passenger seat to make your access simpler.

Regards, Vince