i just discovered that my vibration problem was caused by my flexplate being un-balanced instead of balanced on my EXTERNALLY BALANCED 400. the vibration was slight at certain rpms but what damage could it have caused?
it's got about 500 miles on the engine with this flex plate on it.

It will shake the engine apart. I've seen damage to the bearings.

flexplate being un-balanced instead of balanced on my EXTERNALLY BALANCED 400. .

Dave,
An external balance 400 SHOULD have an unbalanced flex plate on it, not a balanced one ??? If your 400 is external balance there should be a weight on the flexplate. Are you sure this is your problem ? One thing it could be is if there is no dowel pin in the back of the crankshaft to make sure the flexplate is installed in the correct position.

By the way, an incorrect flexplate will be quite a bit more than a "slight" vibration.

The 383 uses a 400 flywheel and dampner, it is externally balanced, I would recommend having any motor pulled down and look at the main bearings for damage.The GM factory 400 were never internally balanced, and their external balance worked out well for their purposes

i just discovered that my vibration problem was caused by my flexplate being un-balanced instead of balanced on my EXTERNALLY BALANCED 400. the vibration was slight at certain rpms but what damage could it have caused?
it's got about 500 miles on the engine with this flex plate on it.

The wrong flex plate on the motor will shake your teeth out, you can't drive the car:yes::yes::yes:

I suspect you had a crap external flex plate that was poorly weighted or like Bill said, the dowl to correctly install the flex plate is missing.