The mysteries behind engine balancing?

Anytime you change the combination inside an engine it should be balanced. Will it blow up if you don't? Could be. A quote from 69 CUDA 440

Stock Piston Weight (273/318/340/360)
Stock 'Cast' Piston Weight Measurements

* 273 (2-Barrel).........530 Grams
* 273 'Commando'......569 Grams
* 318......................592 Grams
* 340......................719 Grams
* 360......................584 Grams

* Speed Pro 'Forged' Replacement Piston for a 340 (723 Grams)