2 hours before grilling, place a cedar grilling plank in a 9 x 13-inch rimmed pan and add enough water to cover the plank by 1 inch; weigh it down with a heavy mug or two so it stays submerged and set aside to soak.
30 minutes before grilling, move the soaked plank to a large rimmed baking sheet. Trim the salmon ends if needed so there is about 1/2 inch of bare plank at each end. Place the salmon, skin side down, on the plank and fold the thinner tail end under slightly if needed to fit.
In a small bowl, combine the orange zest, orange juice, olive oil, brown sugar, chipotle powder, and cumin; stir until smooth.
Brush or rub the orange-chipotle mixture evenly over the top and sides of the salmon. Season with kosher salt and freshly ground black pepper to taste. Arrange the six orange slices on top of the salmon.
Preheat your grill to medium-high (about 435°F / 224°C). Once hot, place the plank with the salmon directly on the grill grates and close the lid to trap the smoke.
Cook undisturbed for 20 minutes. The plank will smoke and may char; keep a spray bottle of water on hand to lightly dampen the plank if flames get excessive, or reduce heat or move the plank to indirect heat if needed.
After the initial 20 minutes, continue cooking and check every 3–5 minutes until the salmon is caramelized on top and flakes easily with a fork; total cook time will be roughly 30–35 minutes depending on thickness.
Use a large spatula to transfer the plank with the salmon to a clean rimmed pan or platter. Serve directly from the plank and offer the orange wedges for squeezing over individual portions.