Bring 5 quarts of water to a boil in a large stockpot over high heat. Add 1 tablespoon of salt.
Add the macaroni, and stir. Cook for 2 minutes less than the time recommended on the package, stirring periodically to keep the pasta from sticking. (The pasta will continue to cook in the Traeger.)
Drain well, and transfer to a large mixing bowl.
Meanwhile, melt 8 tablespoons (1 stick) of butter in a medium saucepan over medium heat. Gradually add the flour and mustard, whisking constantly. Continue whisking for about 2 minutes, being careful that the mixture doesn't begin to brown. (The butter/flour mixture will bubble.) Gradually whisk in 1-1/2 cup of milk; whisk continuously until the mixture is smooth.
Reduce the heat to medium-low and stir in the Velveeta, one-third at a time, until all the cheese is incorporated and melted. Add more milk if the cheese sauce seems too thick. Season to taste with salt and pepper.
Pour the cheese sauce over the pasta and stir gently with a rubber spatula or wooden spoon.
Butter the roasting pan or casserole. Pour the macaroni and cheese evenly into the pan. Sprinkle the Cheddar cheese on top.
Melt the remaining 4 tablespoons of butter in a saucepan. Add the breadcrumbs, and stir to coat with butter. Spread the breadcrumbs evenly over the top of the macaroni and cheese. Dust lightly with paprika.
When ready to cook, start the Traeger grill on Smoke with the lid open until the fire is established (4 to 5 minutes). Set the temperature to 350F and preheat, lid closed, for 10 to 15 minutes.
Bake the macaroni and cheese for 45 minutes to 1 hour, or until the mixture is hot and bubbling and the breadcrumbs are golden brown. Turn the pan 180 halfway through baking. Serve hot.