Yeah, doesn't matter too much actually, the card will run fine in any of those slots. x8 isn't slow enough to make single card performance suck too badly.
In the future though, place the card over your PCI-E slot as if you were going to plug it in, then just eyeball which shields to take out. Not too hard, and makes it easier to figure out which slots need to be removed. I do it like this all the time.