[QUOTE=Gusaroo]Guys, please school me, I wondered about this as well.

Common sense to me says that "externally balanced" means there is weight either added or removed to the flywheel to balance the internal engine spinning components. For example, A 400 FW may have FW weight added at the 2 o'clock position, whereas a 454 FW may have weight added at the 6 o'clock position.

Just because the FW's are interchangeable, does not make the engine correctly balanced... true?

In the case of the 454 flywheel I have there is extra weight added in comparison to a 283 flywheel which is what I was comparing against.
It is an extension of the counterweight.
As I stated before, to my knowledge, GM used automatics behind every 400 sbc produced and therefore if you wish to run a manual transmission behind a stock 400 sbc you have to use an externally balanced flywheel.