It doesn't matter if your car is a positive ground, if you put the red (+) lead of your meter to the (+) terminal of the battery and the (-) lead of your
meter to the (-) terminal of the battery, you should read +6.25 volts. 6.25 volts is a reading of a good battery. Leave your test leads connected,
start the engine and move the carb linkage to off-idle at about 1500 RPM and your meter should read about 7.10 volts +/- if your generator is
charging the battery and the voltage regulator is set up correctly. The amp meter in the dash should be on the + side of zero at this higher RPM.
Also, from looking at your photo of the engine bay, the heater duct from the back of the heater to the firewall is available. Someone who advertises
in The Plymouth Bulletin is reproducing them for your car.