The balancer/damper and flexplate are specific to the '73 cast crank 340, with a specific amount of imbalance to help the overall crank balance. If you get the right parts, it will not change the overall balance.
You can likely get by with the flexplate with a careful inspection at the hub for cracks, BUT the B&M flexplates are about $100 and are a bit of a good insurance policy. What you use and can get depends on the tranny model and if the torque converter has a weight on it or not.
The damper being replaced with a better one is a good investment IMHO, 'specially with repeated high revving. The old ones have hardened rubber that can slip or break out, and won't properly damp crank vibrations anymore.
