Clean and remove stems from mushrooms. Cook bacon until crisp, drain, crumble and reserve. Finely chop green pepper and onion. Sauté onion and pepper in 1 stick butter. Stir in about 1 cup of breadcrumbs. To this stuffing add the drained canned clams and bacon. If the stuffing is too dry, add some of the remaining butter; or if too moist, add a little more breadcrumbs. Stuff cleaned mushroom caps with the stuffing. Place in a shallow baking dish about 2 inches apart. Cover with aluminum foil and bake at 350 degrees for 35 to 40 minutes. Best served warm.