Cook pasta shells to al dente in salted water. They will cook a little more in the soup and absorb liquid from it, so be careful not to overcook.
Melt butter in a Dutch oven or large pot. Add celery and onion and cook until celery is soft.
Sprinkle flour on top of celery and onion. Stir it into the butter and cook 1 minute.
Gradually whisk in chicken broth and milk. Bring to a simmer. Add dry mustard, Worcestershire sauce, garlic salt, black pepper, and cayenne pepper. Continue to simmer until it thickens some.
Add cubes of Velveeta and stir until melted.
Remove from heat and stir in cheddar cheese.
Stir in cooked pasta and serve.