There isn't much to the heaters in these cars, so it doesn't take much to make them work. Anyway, if you want to switch to headers, then you can remove the heater blower and the tubing. The left side block off plate can be found here on Pelican:
Pelican Parts - Product Information: PEL-SM-LEFT
That being said, it is not recommended to run your current exhaust without the heater system intact. The heater system acts as a cooling system for the exhaust by constantly pushing air through the heater boxes and cooling the exhaust pipes. Remove this cooling effect at your own risk!