Add 6oz (about 3/4 cups) of priming sugar to your bottling bucket if you are bottling. Sugar drops will cause too much carbonation for the Irish Stout.
Instead of using 3-4 drops of Lactic Acid to achieve the Guinness "tang", you can try this method I found from another brewer:
Four days before brewing put two bottles (approx. 24 oz) of Guinness in a bowl. Let it sit out on the counter at room temperature so it can sour. On brew day, when you have 30 minutes left in your boil, put the sour Guinness on the stove and boil it for about 10 minutes. At flameout, add this to your wort.