The higher compression, the higher octane required.
With 12:1 you'll have to run straight, leaded race fuel - at least 110 octane.
Another option is the 415cc bore kit. This will create quicker reving power than the 440 kit, and some riders like it better. The 440 kit is best for torque and smooth power. The 415cc for rpm and quick power.
In a drag race on pavement, the 415cc kit will beat the 440cc kit. But in mud, the 440 kit will be better.
Hope this helps you out a little!