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if you're doing a turkey breast as well, i'd put the ribs on top, nothing scientific behind that, just how i do it, if I have something big in there with ribs i'll always put the ribs toward the top, just watch temps and you should be fine either way though. And if you foil your ribs i'd leave them on top and then if you're worried about it you could switch them to the bottom once you foil em..you'll be fine.
FYI - I have seen a lot of discussion here about not putting poultry above any other meats to avoid contamination of the meats below. There are folks that swear by this and folks that don't. Some of them will probably chime in.
I would suggest doing the poultry below the ribs for safety reasons and because I typically do birds at higher heat and I would assume the closer the meat is to the flame the higher the temps would be. Also, the drippings from the ribs would probably flavor the bird nicely. Just my opinions though.