Remove stems from mushrooms, and chop stems. Cook chopped stems and garlic in 1/4 cup butter in a medium saucepan over medium-high heat until all of the liquid evaporates.
Drain clams, reserving 2 tablespoons liquid. Add clams and reserved liquid to cooked mushroom stems. Add breadcrumbs and remaining ingredients; stir.
Brush remaining butter on mushroom caps; place mushrooms, stemside up, on a lightly greased baking sheet or jellyroll pan. Spoon stuffing evenly into mushrooms.
Broil stuffed mushrooms 5 inches from heat 10 minutes or until lightly browned.