In a large pot on medium high heat, add 6 cups milk and 1 1/2 teaspoons salt. Add the uncooked pasta shells, return to a simmer, reduce the heat to medium-low and cook, stirring occasionally, until the pasta is al dente, about 9-10 minutes.
Reduce the heat to low and add cheeses, butter, mustard, cayenne, and remaining 1 1/2 teaspoons salt. Stir until the cheese is fully melted and really melts into the sauce, let stand for a few minutes to thicken. Add remaining 1/2 cup milk as needed.
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