My bet is that there are three belts on [under] the couch and two in the dinette. Many times if the coach was previously owned by an older couple the belts fall down behind the couch or under the dinette becuase they are never used or needed when in underway.
Our 345 was the same as 87MH. We have three kids and were pleased to "dig up" the belts. With the barrel chairs we have seating for 7 in the coach plus 2 up front. You should be the same. The table should fold down and sit in between the two seats...then the dinette back cushions should slide down covering the entire "bed". The curved part should not make a difference.
If there are no belts: it's a pretty easy installation. Any RV dealership could do it for you. Your surrounded by a number of good RV dealerships but and not too far from either the AS factory or Paul Sherry RV's (large AS dealership near Dayton).
BTW - the 360 is my dream coach. Let us know how it turns out.
1986 Airstream 345 Classic Motorhome