How do I identify flywheels at a swap meet? I'm looking for a manual flywheel for a '71 460 motor. It's my understanding that this motor takes an internally balanced flywheel.

1)Where on the flywheel is the part number?
2)What part number do I look for?

It is also my understanding that I could use a 390 flywheel (it supports a 12" clutch.

3)What part number would be on a 390 flywheel?
4)Are there any identifying physical characteristics on an internally balanced flywheel versus an externally balanced one?