Jeff, I would apply a layer of plywood (screwed to wall studs) over your drywall before nailing or screwing your pallet boards. This gives you a nailable base to attach anywhere on the wall. Use a studfinder to locate studs. They will likely be 16" or 24" on center and next to windows, doors and outlets. Also along top and bottom of wall. You should never hang anything of weight off of drywall. Don't make the boards uniform lengths. Mix it up!