I fitted a 500 liter tank to a Connect Van about 4 years ago. The tank ran long ways. It left a gap down each side which was ok for folding ladders and still had sufficient room at the back for a hose reel.
The front 2 bolts securing the frame was over the rear passenger footwells in the people carrier. There is a small gap which provides access from behind the front seats. We used some 25mm x 5mm lengths of flat bar as spreader plates as the gap is quite small.
The back of the tank frame was just above the rear axle and just behind the fuel tank. We used much bigger spacers on the 3 rear bolts.
If you are using bolts down the length of the van to secure the sides, then you have the diesel tank to contend with. I decided that these bolts weren't going to be that critical when used in conjunction with the van. So I drilled holes through the floor and fitted stainless steel nutserts.
http://www.ebay.co.uk/itm/KNURLED-STEEL-THREADED-STEEL-RIVNUTS-NUTSERTS-BLINDNUT-RIVET-NUTS-INSERT-/191625310115?var=&hash=item2c9dc22fa3:m:mbJY077iLT0lrQy_JFKdK0A( Nutserts require a special fitting tool. Our local motor factors used to loan them out to those he knew.)
https://www.youtube.com/watch?v=FFdk4M4Jc4cThere are more basic tools as well. The bolts need to be short as there isn't much of a gap between the floor and the top of the diesel tank. (Careful when drilling as well due to the tank.)
If the tank is fitted across the van then the main bolts might well end up either side of the chassis, so spend lots of time underneath and plan out your fitting properly.
There is a formula to ensure you don't overload an axle. To help your your vans payload is 825 kgs.
The best way we found was to follow the ridges on the van floor and find the same ridges from underneath. If you need to drill, make sure and then doubly make sure. Then drill a tiny pilot hole, say 2mm. If it isn't exactly right you you haven't done too much damage.