I never noticed that problem in the '70 Barracuda I had in Calif, but that was just a stock/original318 car.
If you ant it all to look stock, how about modifying the heater core inlet so you can run a heater hose through the firewall, to a hidden valve, then to the core?
May be too much surgery to the box to accomodate all that.
Maybe take a look at a fuel shutoff valve and adapt it to a hose if stock appearance isn't desired; they're less clunky-looking.
Or use a short length of that black foam pipe insulation to disguise a valve.