You're correct that all the parts are internally balanced, BUT, the assembly was balanced to the weight of a 273 piston. I'm willing to bet those .060 over 340 pistons are a lot heavier than those standard 273 pistons that it came balanced with from the factory. Using those 340 pistons with the other 273 stuff will create an out of balance issue.

OOPS- gotta get back to the honey-do list.