One thing that freaks my wife out about the RB 50 layout is the fact that the rear bench seat only has one shoulder belt and the other is a lap belt. We have kids that are still in booster seats (ages 5 and 7). The lap belt is proven to not be safe in an accident and will actually cause more problems in some cases. This was a big concern for us and I know the Westfalia people have made a shoulder belt retrofit package for this reason. However we don't have anything like that for the Ford nor that will work with the cabinets on the one side so no place to mount an upper shoulder belt.
I found something that seems to do the trick and was designed for seats with lap belts only. We have an advantage as our SMB came with the latch system installed from the factory on the original build sheet. I am sure that you could retrofit the rear attachment point somewhat easily even without the latch system. For kids 66lbs - 168lbs.
The company and product is here;
You can't order directly from them however but if you go to their distributors site they may have one in your state. I called the company and asked where to buy it if I didn't have one in my state and they gave me this number for EZ Way Safety Services (800) 543-0575. They sell it for $75. I ordered one today and will let you know how it works. Just a note they are used to selling it to organizations and schools so tell them you are an individual and there should be no problem.
Here is their website info;
Hope this helps you as I have researched this topic extensively on the web and they all point to this harness system as the best way to make the lap belt safe with kids. The Westfalia boards talk about this issue in depth as their vans also are lap belt only.
Also I found this company through research as I am not affiliated with them what so ever... etc etc. Will post up review when I get it.