I need some help here. I have a 1966 Mustang 289 2V, car. I recently swapped the C-4 for a T-5 I had some major clutch chatter. I'm using a T-5 from a 91 GT Mustang. A 10" disc, I bought a used Flywheel. Put a New Flywheel in today, NOW I have engine vibration that you can believe. Yes the flywheel is in correct, 157 tooth, new from CJ Pony parts.
What else can I do??

 

Clutch chatter has a variety of causes. Most likely it was caused by the used flywheel. You could probably have fixed that by having the flywheel resurfaced.

With the new flywheel you have significant vibration. Most likely, you have the wrong balance. A 1990 V8 uses a 50oz balance weight. A '66 V8 uses a 27oz balance weight.

Your best bet is resurface the flywheel you had on there first, and install that.