Externally balanced engine troubles

[JMD]

Hi all,
Started my rebuilt 454 (Originally out of a '78 chevy pickup) for the first time today and found that it has a fairly violent shake that increases with engine speed. The only thing I can think of is that the PO put an internally balanced fex plate and harmonic balancer when they should be externally balanced pieces.So my questions are...
1. How can I tell for sure whether or not my flexplate/balancer are externally balanced?
2. Can I just buy an exterally balanced harmonic balancer/flex plate and install them? Or will I need to get the whole rotating assembly balanced as well?
Thanks in advance for amy help
-Jordan