I was the one who resurrected the model 20 rear end discussion but I'm still not entirely clear on how it all works.
* If you have an AMC FACTORY rear housing and carrier, you can use any proper AMC FACTORY gear set. With the exceptions Tom Bunsey points out.
* If you have an aftermarket rear, you ask the manu what gears fit.
* If you have a factory rear, and buying an aftermarket gear set, you ask if it fits a factory rear.