I added a fold up bench seat to my truck. As well as a lot of cabinets. For the bench seat I ended up mounting it over the wheel well area since that was the location I wanted it. To make it safe, I put a 1/4" plate of steel over it and then bolted that to the cross members below. The seat is then bolted to the plate in some places and through the plate and the cross members in other places just to make it safe.
For mounting other items like cabinets or sink, etc, I ended up really liking to use pop rivets and rivet nuts. I put them right into the upright supports when possible, and when not then just into the sheetmetal. The M109 boxes also have a ton of 1/4" thick steel bars that I drilled and tapped and then screwed items into them (you can see some of the uprights and reinforcement areas in the picture showing the gutted box). The boxes definately have plenty of reinforced areas to mount into. You can buy taps, or rivets and rivet nuts at the home improvement stores, or my favorite store... mcmaster.com
http://www.mcmaster.com/#standard-rivet-nuts/=mlgv39
OH, and my other favorite thing to use is Vulkem. Its a polyurethane construction adhesive (Caulk). I used it everywhere on the truck to seal the box joints, and the holes that I drilled through the floor. It stays flexible and is some tough stuff.... its great. You can get it in lots of colors at home depot and other places.