I did alot of research about this same problem. Because AMC v8 engines are externally balanced it's important to get a
AMC 304 flywheel for a
AMC 304 or to have the rotating essembly balanced by a machine shop with whatever flywheel you use.
The flywheels for a inline 6,
AMC 304 ,360 all look the same except for the backside. The different flywheels have the added weight at different locations and the flywheel will only bolt up one way.
I came upon this problem when I got a NOS engine and found out I had a brand new never run flat faced crank that was only avail. 1970-71 and was told by 2 different places that they had not seen a flywheel for that crank in the last 3 years and this was from places that specialized in AMC motors.I changed out the crank and used a 360 flywheel. Can't feel any vibrations but would have had the motor balanced if it was still apart.
By the way, If it's only your ring gear thats bad,you can heat it up with a rosebud tip on a torch and replace it.Comes right off and I think I have a good one off a 6 cyl. flywheel that I have.
Sorry but I can't remember the web page I got the info from but they had the casting numbers and a description of what each flywheel had on the back.