Heat a grill to high heat. Place 4 tbsp butter and 3 lg garlic gloves, chopped in a small saucepan over low heat. Melt the butter and infuse it with garlic. Turn the heat off and leave the saucepan on the burner. When the grill is ready, place a disposable aluminum pan (9 x 13 inch) directly on the grill grate. Add 2 doz littlenecks clams and cover the grill with the lid. Cook for about 15 minutes; the clams will just beginning to open. Add 2 doz mussels to the pan along with the clams and replace the lid. Grill for another 5 - 10 minutes or until all of the clams and mussels have opened. Drizzle the garlic infused butter over the shellfish and sprinkle with 1/2 c fresh tomato, chopped and 2 tbsp fresh basil, chopped. Serve immediately.