Balancing a 360

I checked on both the Summit and Eagle website and it seems that the cast 360 crank is in fact externally balanced. Seeing as I am using the OEM flexplate, torque converter and balancer, the vibration that the engine is giving is normal? I did switch out the pistons but they are nearly identical to the stock ones in terms of size and weight. Seeing as this is my first carburetor powered V8, this vibration may just be normal engine operation that I am misidentifying? Would there be any other telltale signs that I am missing?