First, you can't extrapolate from the experience in Florida to the rest of the country. Huckabee plays differently, in different parts of the country.
Second, a head to head match up would show the real strength of McCain. In 2000, he only pulled 33% of the primary vote and he is about at the same percentage now. I seriously doubt that faced with the choice between nominating the GOP's maverick and Romney that McCain will emerge on top.
Based on my on-the-ground experience in SC and FL, I think McCain would lose in a head to head match. And then there is the Ron Paul factor who also draws from conservative voters. McCain is the most liberal Rep in the race. I just don't see him beating Romney head to head around the country.
Huckabee will hurt Romney the most in the South.