Well, as it happens i recently -did- this.
I added a sideways-facing Flexsteel sofabed (purchased from Camper's World) and then added lap belts. The sofa/bed was as rear in the van as possible, on the driver's side.
(in a 118" passenger van, with the original seats removed).
There were lots of factors:
(a) here in Washington State, you are not -required- to have seatbelts in the rear area, if they were not put there for the seats you're using by the original vehicle manufacturer.
(b) in Australia (and most other places) they may require seatbelts on all forward/rearward facing seats, but they do -not- require belts on sideways-facing seats, both for "lack of data" and the possibilites of severe trauma from the lateral forces.
(c) some juristictions don't allow seating if there are not seatbelts there.
When you buy "add in" seat belts, they come with big washers and stronger, fine-thread bolts for the attachments.
And recommendations for the angle which the belt should rise from the floor, usually about 70 degrees.
In our case (with the movable bed frame) that required cutting holes through the foam mattress to hit the angle.
You can run a plate under the Sprinter floor to spread the anticipated forces across a wider area.
In my case, the places we wanted to mount the belt anchors were "inconvenient" in terms of how they hit the understructure. So i created bridging structures in the van which let me bolt through the floor fore and aft of the underframe. With those, I was even able to use the existing seat receiver attachment bolts (after buying longer, harder versions) for one end of each "bridge". The bridges were short-side steel channel, which just allowed the head of the belt attachment bolt to clear the floor.
Eventually i'll add a second (and perhaps third) set of belts back there. At least the second set will be "simple" drill through the floor just beside where the wheel well's inner wall ends.
I did not do a shoulder harness, since the "stong point" would've put the anchor behind the passenger.
I'm not -thrilled- with the current installation, it was done quickly for a "must have third seat" visitor, and had to be fully removable for our cargo usage.
That trip has passed, the belts were not "tested" by any accident.
If your sofa is in a permanent spot, you can add lots of above-the-floor bracing (such as my "bridge") to help with the "where can we bolt them?" issues
have fun
--dick