Internal and external balance.

If you're switching to lighter parts(piston, pin, rods) you may be able to move to internal balance without adding heavy metal. Internal balance reduces loads on the front and rear bearings due to crank flex. External balance is typical on engines with insufficient crankcase dimensions for properly sized counterweights, like SBF's or just cost savings in general.

I wouldn't put any money into a cast crank. They do drill nice, but 4340 forged cranks are more available than they've ever been. S/F....Ken M