I am balancing my engine 394 internally do I need the flexplate/flywheel balanced too. If no can I buy regular flywheel or should I use orginal one.

 

The flywheel and the balancer must be balanced with the crankshaft as a unit.
The flywheel and the balancer are both deliberately off balance to make up for crankshaft imbalance.
This is known as "offset balance " or "external balance"
This was common to all 57 thru 64 Olds engines, except F-85's.