Will a Stock 489 handle 700hp?

You can spend all the money you want....sooner or later the pinion gear will eat the teeth on the ring gear.....bracing...new caps...still same old unsupported pinion gear.
So I guess by your analysis a Dana 60, 70, or 80 pinion is unsupported? Therefore weak? I've said it many times before, There is a reason why a 9" has three bearings, Because the inner and outer main bearings are a 1/2" apart! without the nose bearing it has no lateral support! But you that want to believe the 9" with it's puny 1.313 Diameter pinion ( smaller than the 741 which is 1.375 ) is better, carry on.