Boil the shells in water with 1 teaspoon salt for 3 minutes less than the package directs and drain.
Steam the broccoli for 4 minutes until bright green and barely tender, then rinse briefly under cold water and drain well.
Butter a 6-quart crockpot and add the pasta, evaporated milk, whole milk, mustard, pepper and remaining salt.
Cook on low for 45 minutes, then stir in the cheddar and American cheese by handfuls.
Fold in the broccoli and cook on low for 45 to 60 minutes more until the pasta is tender and the sauce is smooth.