12.6 KW. What mischief are you up to?
Suggest you go to jaycar.com.au and enter "heatsink" in the search box. Look through what they have available, will give you a good idea.
For instance, a fan type, 105mm wide * 225mm long, is 0.9 degC/watt.
Fan type, 105mm wide * 135mm long, is 1.1 degC/watt.
I think the flat plate formulae are in my voltage regulator book on the bookshelf at work. Google would probably find it.
Amanda