The key to a Guinness clone seems to be about 10% roasted barley and 500 odd grams of flaked barley in 20 litres. Guinness don't use any other roasted malt, and the flaked barley gives the beer magnificent foam. If you can, also grab one of the liquid Irish ale yeasts because they add a bit more character. I did an all grain one a while back and it was damn near identical to Guinness.