Where would an alternative mounting place be for such a brace then, if not in the factory floor mounts?

Definitely a quality suggestion!

 

There really is not one because the FD was no designed to have the diff mount to the floor. I am sure plenty of people get away with this, but Im saying if it was my car I would reinforce the floor with some plate.

 

What about drilling a hole in the floor, and making a sandwich plate style bracket? do you think this would solve the issue? or just put you on borrowed time until it eventually breaks?