You can use a factory fader switch and run the single dash and single rear speaker. That was a factory option for mono radios. A factory fader switch is the the best way to go in order to run two speakers without altering the impedance .You always want to be sure to maintain 8 ohms with these old radios.
When you start hooking speakers up in parallel or in series, you change the impedance. If you drop down to 4 ohms you run a risk of overheating the radio's output transformer.

Greg