While many people use spray foam with great results, consider a few things before you choose to.
1. Learn what "offgassing" means and how it can effect your health in a tight, small box like that.
2. Make sure you understand how it can trap moisture and affect the metal in your van over the long haul. (AZ life would not worry me...but if I were all-seasons everywhere, watch that moisture.
3. The Styrofoam panels, natural fiber and good ole fiberglass have all been used to insulate the custom box trucks. The builders that do the custom stuff(mobile grooming, bookmobiles, mobile MD offices, FBI vans...) like the plain white styro. You can cut it with a razor, carve it and it glues easily to any surface. Many specs do not allow its use in for-profit truck outfitting however.
Allowing the metal to breathe, rather than sealing as some have with plastic wrap (inside) is better for the van and the mold you may avoid. Open air holes in the bottom/top paneling will be kind to you. (Many carpenters try for a sealed situation.)
If your van is not white, having the roof "wrapped" white will make you smile in the summer.
You are going to LOVE what this does to the noise in your van.