Not sure that the slats are actually cedar, but maybe they are. Anyway, I'm guessing that uou are talking about boards with gaps between them, that originally had cedar shingles nailed to them, and I also am going to guess that the cedar shingles are long gone, and that you now have asphalt of fiberglass shingles? If so, vent installation isn't much different than on sheathing, except that you obviously need to watch where you nail. The larger issue though, is that the house very likely was built without insulation, and when insulation was added, what steps were taken? As an example, is it a 2 story or story and a half, and what size rafters does it have? A lot of those houses had knee walls, and when insulation was packed in from the inside, it was by default packed against the roof boards, both in the ceilings and in the soffits, in which case today's methods of venting may or may not be suitable. Just some things to think about. So, to answer you question, not much of a problem to install vents, but you may want to talk with an architect or engineer regarding the most proper and effective methods of venting for your particular application. Other opinions may vary, and that's okay; it is after all a forum.