Internally Balance Cast Crank?

Back in the early seventies my brother had a nice Aussie VC Valiant with 273and he added a edelbrock LD4B manifold, Holley carb and headers and it ran great for a few years.
After a few years he broke a crankshaft. He then put a 1968 hi comp 318 in it with just the for mentioned add ons.
Man that thing was quick. He used to run GT Falcon 351s and regularly knock them off!
As the thing had a standard cam in it and would idle like a std car, when he would line up next to the GT Falcon 4 speed manual his std looking and sounding Valiant would get Jeers but silenced a lot of people after they saw how it ran.
Now back to the 273. That would of had a forged crank in it so it must of been highly unusual to break one of those.