flexplate install

OP, did you give your flexplate AND your 360 damper to the person doing the balance work to be used in the balance process? That normally would have happened, as the 360 is what is known as an external balance engine, where intentional imbalance in the damper and flexplate (or torque converter) are used as part of the counterbalance weight. If they did NOT use these 2 parts in the process, then they internally balanced the engine, in which case you cannot use an original 360 damper.

Let us know, and if it is not 100% clear to you, make the call to whoever did the balance and ask! It's important!