318 build up

The crank is balanced for the stock pistons, stock-type useage.
The ballancer is neutral ballanced,as must be the convertor.
The oem flexplate is universal to all cranks .

If replacement pistons will be of a different material or type/shape that leads to an other than stock weight, a reballance is suggested. If the engine is to be operated in an other than stock manner then a reballance could/should also be considered.
For instance, if heavy forged pistons go in, and with a 3800 stall TC , that crank was never ballanced for that appication, so ......