Just a thought on that-
The rear spring hangar is moved WAY back on the frame from the original suspension attachment. Any weight applied to the frame will split at that point between there and the front spring hangar. The front hangar can't be but a few inches in front of the original pivot. That in and of its self (other components aside) should strengthen the toungue capacity as far as the original frame rail is concerned. Since the origiinal frame is still there, and by all appearances, uncompromised, I'd think that as of the last picture, going with the factory rating would be quite conservative.
Now, once the crane is attached and you add the static, and more importantly the dynamic forces there, you have an undetermined chassis load, based on it's height, weight, it's own independant center of gravity, storage position, etc. It is an unknown for sure. Not having experience with either one, given only the information in this forum, I don't think this is going to tax the frame any more than your well proven approach of having a HIAB mounted at the front of the bed, where the frame is supported by the axles at both ends, but with a much, much larger span.
I don't know enough about either crane to figure out how that actually works out with numbers, I'm just thinking out loud, cursing the rain, and admiring a project that i only wish I had the time and resources to take on for myself...