On a prop plane the prop will act as a very effective speed brake in a dive even at full throttle. If you take the pitch speed (pitch in inches x RPM x 60)/12 x 5280 = MPH and figure on a 40% overdrive in a dive that should give you a pretty decent max speed to work with. That should allow for the engine being driven above it's normal speed and for the negative slip of the prop disc in the dive. So use 1.4 times the pitch speed and see what the load calculators say.

Does that seem reasonable in your estimation Dick?