Preheat your oven to 375℉ and set the rack in the middle position.
Cook the elbow macaroni according to the package directions (minus 1 minute). Drain in a colander and set aside.
Melt the butter in a large nonstick pan over medium-high heat. Add the flour and cook for 1-2 minutes, whisking constantly, until it develops a golden color.
Whisking constantly, gradually add the milk and heavy cream. Once the mixture boils, reduce the heat to medium and continue cooking for 2 minutes until it thickens to the consistency of heavy cream. Remove from the heat and stir in the cheddar cheese, salt, and black pepper.
To a large bowl or pot, add the macaroni, imitation crab, and cheese sauce. Mix until combined. Pour the mixture into an 8-inch square baking dish (see notes). Sprinkle breadcrumbs on top.
Bake for 15-20 minutes or until the breadcrumbs are golden brown.
Notes
Baking Dish: A 9 by 13-inch dish works great for a double batch.