Just to clarify, the aftermarket heater I currently run attaches to the firewall under the dash. Your choice is floor heat or flip a lever and you divert to defrost, two 2" round tubes that point up, like you attach hoses for defrost. The heater hoses come through the firewall.
I'd still advise picking up a '40 or '55 rear heater and installing under the driver's seat, but then no defrost. The '65 and '66 had those wing like things that came through the windshield frame for defrost. I fabbed some sheetmetal ducts that I pop riveted to the underside of the cowl and connected them to the heater with some flex hose, but I'm also running a '73 windshield. Frankencruiser...
GL
Ed