Hello...
I'm not sure, but based on the post on the gear page by cameron:
"When Phil Spector put together a boxed set of his recordings he went back to mono, and that set is a revelation, since all we'd heard on the radio for years were the lame stereo mixes of those tracks."
It seems that the stereo mixes was what was being heard on the radio. But, I'm not sure if this applied to am radio. And, I wasn't even born yet so...I defintely wouldn't know first hand, or first ear.
Some important tips about mixing to achieve a mix typical to 60's music, but also applicable to any genre, depending on what you are going for:
-simplicity with the drums, and minimal layering for better spacing of instruments.
-mono instead of stereo (in some cases)
or -hard panning for spacing... ex. guitar panned hard left and bass hard right for separation.
Although I have read many different posts about ideas on panning, and separation, but not in regards to 60's, however, definitely useful.
This post was helpful :
https://homerecording.com/bbs/showthread.php?p=1326193
Also about panning, stereo, and mono but more in terms of mixing in mono and referring back to mono when mixing. If you can get the the mix to sound good in mono by levels and eq'ng, then once you get to the panning it will just open up the mix even more. Also, someone posted how to mix (pan) a drum set as if they were behind the kit.
https://homerecording.com/bbs/showthread.php?t=145220
- band playing together live instead of recording one track at a time, with the exception of the vocalist
-using your ears, instead of visually mixing with computers etc.
- eq'ng wasn't discussed. I bet very limited eq'ng, and I would say subtractive eq'ng would be the way to go to make room for each instrument in it's frequency range.
I like this article:
http://www.homerecordingconnection.com/news.php?action=view_story&id=154
Reamping:
http://www.stompin-ground.com/products/radial-xamp-hist.htm
Vocals:
http://www.recordinginstitute.com/R2KREQ/excomp.htm
Just some information I've gained from reading up on different posts. I learn new things each day! There's always something else I discover and wonder, hmmm why in the world didn't I know that it just makes sense and seems so obvious, but I guess that's how we learn. Hope this helps

evt