Spaghetti with Chicken Sauce

This recipe is from a cooking for a crowd book.  Quantities should be adjusted accordingly.
Yields: 50 portions

7 oz butter
1 lib 8 oz celery, chopped
1 lb 8 oz onions, chopped
2 oz green peppers, chopped
2 oz pimentos

Saute above in butter until tender. Stir in 10 oz flour. Add  4 3/4 qts chicken stock to vegetable mixture, stirring constantly. Cook until thickened. Season with salt and pepper.  Fold in 7 lbs cooked, cubed chicken.  Cook 5 lbs spaghetti. Serve sauce over pasta.