I think the real issue is the inner pinion bearing. the 742 has a larger bearing and a 489 is even larger. these are Timken bearings that control the thrust on the pinion. the larger bearing will have more control and less deflection. there is a reason why any engine, 361 or larger, came from the factory with the larger pinion. in my opinion a 500cuin engine with a clutch with a dana 60 is the only solution for trouble free driving/racing.