Maz, one of the benefits of a van is that you don't have to compress the bike's suspension by tying it down.
I have 2 chocks in my van. I strap the front wheels in there, and then I attach from the rearsets across the seat to the upper wall in each lateral direction.
The bikes can't go forward or back, and can't fall down..
I think my ramps are 8' long.