How to mark the ribs?

[Ballenxj]

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?

I would make tiny marks that align with both sides of the rib, and make another tiny mark that indicates center. I would do this at both ends of the rib to eliminate any mistakes, or in the event the rib was not square. Hopefully it is though.