High Wind-Resistant Retrofit Construction
Actually, it's not just "tying the roof down to the foundation."  Its more than just uplift. It's tying the roof to the walls (for lateral resistance), then tying the walls to the foundation. 

Where I live, we get 120-140 mph winds every year, but they're "gusts".  Where you live, the winds (tornados...hurricanes) are "sustained" winds. Much tougher to design resistance for hold-downs and lateral resistance. 

However, here we have seismic problems...both vertical and horizontal loads. Structural design is a process. I'd contact a structural engineer (not a civil engineer) or an architect.