How to mark the ribs?

[Matlock]

I am planning on buying a cargo van, using Polyiso for insulation, covering with luan-type paneling (we shall see). 

But... how do you mark where the ribs and holes are, for future additions, like a shelf or cupboard?  I'm thinking that there must be a way, but I can't really figure out how to do it.  Any tips?

Once I determined where I was going to locate (or use existing) frame for fastening to in the future I took photo's holding a tape measure showing the distance to the center of that mounting point from a fixed surface.
In this photo it tells me the distance from the finished floor to the center on an installed 1x2" board that runs all the way around the inside of my Box Van.
   

Any added installation I do after my paneling is up is easy as long as I reference my photo's.

Richard