Preheat an oven to 350 degrees F (175 degrees C). Grease 9x13 inch baking dish.
Cook the pasta uncovered, in salted boiling water, stirring occasionally, until the pasta has cooked through, but is still firm to the bite, about 10 minutes.
Drain well and place shells on a sheet of wax paper to cool.
Combine the roast beef and the gravy in a large saucepan and place over medium-low heat.
Cook for 10 minutes or until heated through.
Use a slotted spoon to fill each shell with roast beef, draining off as much gravy as possible.
Place the shells in the greased baking dish, and top with the remaining gravy.
Sprinkle the Cheddar cheese evenly over the stuffed shells.
Bake, uncovered, until the cheese is melted and the filling is hot and bubbly, about 20 minutes.