Add a quarter lb of acidulated malt, and you'll be even closer to Guinness draught. Check out this thread
, which discusses the draught recipe to a good extent. One step further, you could set aside a few pints of your wort, sour it, pasteurize it, then add it back to the finished beer. Supposedly, that's what Guinness does.
I brewed a stout pretty close to this one and I'm pushing it through a nitrogen stout tap. Now that's about as close to Guinness draught you can get at home!