How do I tell if my 340 is internally balanced?

I'll have to think about that a while. It was never explained to me that way nor have I ever read it done that way. Thanks for teaching me something new. Never too old to learn.
IMHO... it may help to think about the B&M flexplates with the 'bites' out of one side. That missing 'bite' of metal is the same effect on balance as adding weight on the exact opposite side.