AMC engines have a few different choices in exhuast systems. The main thing is that you plan for the type of use the motor is intended for. Most rock crawlers need low end torque. This is best implented by the use of manifolds and a single exhaust system. And while the stock exhaust will work for stock motors, one can still get a better system made that will increase power without suffering too much low end loss. The main thing is that you do not choose a restrictive muffler, but one that compliments your motor.
The old theory of the straight through glass pack being the best choice for flow, has long been disproven. The glass pack itself has many flaws that actually CREATE more turbulance. This is caused by two things. First the insides of glasspacks have small protrusions. It is these small lips of metal sticking out that are supposed to gather gasses and force them into the actually fiberglass packing. These lips create a noticable amount of turbulance.
Note. Newer designs do not usually have the lips protruding into the flow of gasses. And create a smaller amount of back pressure.
Second things is that there is no room for exhust expansion. There is a small amount of room in the glass packing, but not enough for practical purposes.
Now there are makers like Borla that offer a product that resembles the traditional glasspack. But these designs do not have any protruding material, and also have a much larger amount of room for expanding exhaust gasses. And now there are mufflers with stuff like ceramic and stainless steel wool. And many of these work quite well. But they still are going to give a sound that resembles the traditional glasspack. I do admit that with actual flow testing and research, that the sound and performance qualities have gotten much better.