Hello there,
I installed a ball-valve on my 1982 when I replaced the hose going from the intake manifold to the heater core. The heater shutoff valve was questionable, so i didn't even re-install it. Simply place the ball-valve anywhere in the hose. (just remember that you'll have to pop the hood and open the valve to turn on your heater).
here's a link to a valve similar to what i used. Any hardware store will have them.
http://www.homedepot.com/webapp/wcs/...ctId=100039901
I did it to my car, and i love it. I get zero heat on the floor boards, and if my heater core ever leaks, I can prevent the fluid from leaking into my interior.
Good luck!