The mysteries behind engine balancing?

Back in the 70's the my 273 engine balance included a small weight welded to my clutch pressure plate by who ever did the work in Salinas, CA. About a year later, when I sent the pressure plate back to the manufacturer for a rebuild, I was told my warranty would be voided if I ever had anyone weld anything on the pressure plate again. What I know about engine balancing is each piston, pin, and rod assembly should weight the same as the others. What I don't understand, is how the same crankshaft with the same counterweights can be used in different motors. For example, the 273 and 318 motors use the same crankshaft, but don't the pistons differ in weight for those two motors? So how can the same counterweight on the crankshaft compensate for that difference? The other mystery is what else is done to balance our motors, besides equaling the piston/rod weights? Is the crankshaft spun like balancing a tire? And what was the purpose of that small weight added to the pressure plate? Was the clutch assembly being balanced separately from the motor? Obviously the balance job was effective, taking into account a best ET of 12.72 at 108 mph by a 273 with a 600 CFM carburetor. Thank you for any insight.