If you can obtain a tank of 75% Nitrogen and 25% CO2, do that. Lookup "AirGas", that is where I got mine. They just had to special order it. That is the mix Guinness uses. It will be a nitrogen tank, so different valve, so you'll need a nitrogen regulator.
Keep in mind you must have a very low carbonation volume, like 1.2-1.6. It does not take much to achieve that. Then just hook the gas mix up to the keg at about 30psi at 38*F through the Stout faucet and you're good to go.