^^^ +1
The mixture will greatly depend on where you are located and the season. For hotter more dry areas closer to 100% H2O will be the choice because it will evaporate and remove more heat. In colder more humid areas H2O will not evaporate as easily, so use 50/50 or 100% alky to cool the charge down.
As dudical26 said plan on using a lot of "injection fuel" to effectively cool the charge down. Also if you do plan to use more methonal, try to use Stainless piping to avoid corrosion. Although corrosion will still happen with alky.
Another option is look to switching if your able to E85 fuel, it naturally colder than gas and will aid with the higher octane. Although in Japan I believe your blessed with 100 octain or so from the pump!
Good Luck with your build!!!
Evan Smith