Cook pasta according to box. When there's 1 minute left, add bok choy. Drain, put back in pot and set aside.
In a large saucepan heat milk and butter over medium high heat. Add salt and pepper and stir. Add Velveeta and cheddar cheese, stirring until it's all mixed and creamy. Remove from heat.
Add creamy cheese sauce to pasta and bok choy. Stir so all that sauce covers all your shells.
Take bacon and crumble it over the pasta shells.
Serve in bowls, garnish with chopped up scallions.