IIRC, my local auto parts store had a heater core for my 66 for under $19 and surprisingly it was in stock, with lifetime warranty. I didn't need to remove the heater box from the car (just undid the clips and opened it). The plenum and pass seat were already removed. Good time to replace all the hoses in addition to the long heater hoses. (Make sure the new lower radiator hose has an anti-collapse spring in it, they usually don't.) Get some fresh strip caulk to place around the firewall where the heater hoses enter the pass compartment. Not a pleasant task, but not hard.