Vikki: The factory flexplates/flywheels are all balanced to the same specification, but not neutral. (That is why they only go on one way). Consequently, the factory engines are balanced to work with the factory flexplate.
When you have an engine builder balance an assembly, you should tell him what you want. 1) A neutral balance assembly without the flexplate or balancer. (now you need a neutral flexplate) 2) Balance it with factory flexplate, but do not let him touch the flexplate. (Now you have an engine balanced with all factory flexplates) 3) Worst case is he thinks too much like the Chevy guys and asks you for the flexplate and balancer, then modifies them during the balancing. Now you have to keep those components together.
Aftermarket companies will usually list in the specs if the flexplate is neutral balanced or offset balanced for the factory engine. If the company is not sure, stay away.
