Look for mushrooms similar in size so they will cook evenly and at the same rate. If you live near the ocean, splurge on jumbo lump crabmeat, or even less expensive crab claw meat to use in place of canned.
Rinse mushrooms, and pat dry. Remove and discard stems. Combine mushroom caps and dressing; cover and chill 1-2 hours. Drain well.
Preheat oven to 375 degrees F. Combine 1/2 cup breadcrumbs and next 5 ingredients; stir well. Spoon crabmeat mixture into mushroom caps; sprinkle with remaining 1/4 cup breadcrumbs.
Place mushroom caps on a rack in a broiler pan. Bake at 375 degrees for 12-15 minutes or until browned.