Size of the drum and critical mass RPM diffences between them. I don't believe it's 3x, it was the first gear ratio in trans.
727 critical mass is around 12K which just about any good running BB can hit. Only about 5000 rpm
904 critical mass is around 23K rpm IIRC. Gotta spin that engine up to some real RPM to hit it. Most street strip stuff never gets there.