Soak the cedar planks: in a 9x13 nonreactive dish combine hot tap water and 1 tablespoon salt; fully submerge the planks and weigh them down. Soak at least 2 hours (or overnight), then drain and pat dry just before grilling.
Make the wet rub: in a small bowl whisk together 3 tablespoons Dijon mustard, 2 tablespoons maple syrup, 1 tablespoon balsamic vinegar, 1 tablespoon lemon zest, 2 teaspoons minced rosemary, 1 teaspoon smoked paprika, 1/2 teaspoon garlic powder, 1/2 teaspoon dried oregano, 1/2 teaspoon salt, 1/2 teaspoon pepper, 1/4 teaspoon onion powder, 1/4 teaspoon dried thyme, and 1/4 teaspoon chipotle powder until combined.
Prepare the salmon: place the 46 oz salmon fillets skin-side down on a baking sheet or cutting board. Brush the top and sides evenly with the wet rub. Let rest at room temperature for 15 minutes or cover and refrigerate up to 4 hours; bring to room temperature before grilling if chilled.
Preheat the grill to medium-high (about 400–450°F with the lid closed). Keep a water bottle nearby to spritz any flare-ups.
Grill the salmon on the planks: transfer the dried cedar planks to the grill grates and place the salmon on the planks. Close the lid and grill until an instant-read thermometer registers 130–135°F for medium (about 12–15 minutes), or cook to your preferred doneness. Spritz the plank edges with water if they begin to flame.
Rest and serve: carefully transfer the planks with the salmon to a baking sheet using a large metal spatula or tongs and let rest 5 minutes before serving.