..in a world of modern 200 watt stereos...
what was the output of a '70 AM FM radio?
to add 2 rear speakers should they be 8 ohms in llel or 4 ohms in series?

Speakers are 8 ohm & output I think, was around 5 watts per channel. Contact Gregg Ward who is a Moparts member who specializes in restoration of Mopar radios & did a great job on my am 8 tracks.

Yes , the 1970 AM FM was rated at 5 1/2 watts . Factory speakers were 8 ohm. You did not mention what body you have, but with the B body or A body (mono radio) you will be running a single dash speaker and a single rear with factory fader switch. However if you are going to not use the factory configuration and you are going to run two speakers in series (without the factory fader switch)they need to be 4 ohm each to result in 8 ohm impedance .
To wire two 8 ohm speakers in parallel would result in 4 ohm impedance
and that is VERY BAD .
You never want to run 4 ohm total impedance with these old radios.
You will burn up the radios output transformer over time.
If you are talking about an E body (stereo) you simply use the factory 5 speaker harness with crossover box and factory stereo fader switch ,and plug everything in where it goes ,using 8 ohm speakers.

Greg

...tx so much Greg.
it is an A body and the wiring and fader sw. are all there.