I'd use birch plywood, 1/4" or 3/8" and knot-free softwood bearers [ and for a van BB grade would be more than good enough]. Strong, light, easy to cut and shape, the 1/32" thin veneers and lack of voids mean you can sand up or rout the ply edges smooth without lipping or the sharp edges of metal. Chipboard or MDF is useless for racks and shelving IMHO. The weight/stiffness ratio of Birch ply is as good as Dural- and hauling excess weight about is getting expensive. Plus, little risk of setting the van on fire with a welder! Pop rivetted or bolted ply or softwood longerons, fixed to the existing van internal struts at strategic points, mean a strong chassis for a racking assembly can be rapidly made. I would avoid drilling the floor! Ply-offcut doublers can be glued in at stress points. I now use 'Titebond III' as my favorite woodglue, + 18g brads / woodscrews for structural joints, [good for at least 800 psi, water clean up, 100% waterproof cured, rapid grab]. A coat of water-based varnish to keep the grime of everyday work off the wood is a good idea. Tip: Boxes must be stable on the move & not be overfilled or they will self-empty when you hit potholes or kerbs!
Doubts? Here's the biggest aeroplane that ever flew, built almost entirely of laminated birch:
http://www.youtube.com/watch?v=R5XSESxKfKE 
