NEVER cook clams to recook clams in a recipe. Freeze the chowder clams. Then put them in the sink and run tepid water over them. The shells will gape. Put a regular table knife between the shells and pop them open. This is much easier than trying to hand shuck them. Then cut the frozen clam into 6 or 8 piece and put in a stainless steel bowl to defrost. Add the clam meats to the recipe with a slotted spoon. The clam liquor can be also added to get more clam flavor. Cooking them first only makes them tougher in the final recipe.