There are also cross members running between the chassis rails on Vivaros.
Use these as well as the chassis and where you cant use eother chassis or cross members use large 6mm stainless spreader plates beneath the floor. You are fine to go through the chassis but don't rely on just nuts or nuts with washers. Again use spreader plates beneath the chassis and cross members.
You will have to drop the fuel tank to make sure the tank is properly secured.
We did a Vivaro on Friday. A 650 upright and we used 20 bolts evenly spread around the cage bottom. 6 on each long side and 4 on each short side.
On the bolts through the chassis and cross members we used stainless steel inserts in the holes into which the bolts were placed to strengthen the whole fitting. This is a LOT stronger than just through the chassis. Also don't forget to underseal each plate/bolt or you run the risk of rust.
Andrew
