For starters, my disclaimer: I didn't do it!
Here are few photos showing how the stock front rotors were swapped out with 12.18" x .812" vented rotors. This is the setup that was on the original car when I bought it. It works great for my needs so I haven't changed anything. If I were building a road race car I might think twice about some of the details.

Here is the big rotor mounted to the stock hub... It looks cool, but notice how the hat section is flush with the rotor surface?

The rotor is effectively mounted backwards on the hat which theoretically will restrict airflow through the rotor. I have never had any overheating problems so I can't say this is a problem, but to mount the rotor this way (when doing a brake job) I had to machine the backside of the mounting tabs on the rotor to get a flat surface.

The hat section used is a 1.75" piece. To keep the track width correct, the back of the hub was turned down about 1/4".