@tFtR - I fitted these to my van
Marked out the footprint - the tricky bit was taking a bead up the inside of the van window, then copying it down the outside to ensure I had it all lined up and was not hitting any crossmembers in the chassis underneath. I spent AGES doing this and in the end it worked...
You can see the "legs" attached now to the floor in the back ground - they have a long flat plate on the inside, which doubles as a template and each bolt under the floor has a singular spreader plate.
These are the seats http://www.vanbitz.co.uk/A2-High-Back-Double-Minibus-Van-Seat/7119.htm
The red frame that my OH is sitting on is the steel frame for the bed, I got that made to measure in 2" box steel at my local fabricators which only cost £120!
It now more regularily looks like this