I'd try to get it filled as soon as possible. If you're going to let it sit DO NOT use a sulphur stick in it. It will explode. If anything, I'd fill it with water and citric acid. I used 3 cups citric acid for my 59 gallon barrel. The idea is to get the pH below 3 or so, so no bacteria or mold grow. Since it was freshly emptied and was a booze barrel, you have some time before that happens.
On a side note, you're probably going to want to get some of the booze and oak flavor out before you turn it in to a sour barrel. Have you thought about anything else to put in it first?