Most marina's (so i've been told) will allow a couple feet over the stated slip size to accommodate overhangs, as mentioned. In my marina, my Cat 27 is allowed to occupy a 25 ft. slip... with the outboard's overhang another foot could be added, but I've been there for 24 years and had no complaints. So I would suggest asking the marina manager what is allowed. The deal is your rent is based on boat length or slip size. whichever is greater.
If it were me, I would take the slip, no need to get too nit picky, because they are going to charge you for the 30ft anyway.