Good question, I ve wondered that myself. I use frozen clam bellies for chum and take out about a dozen and half or so fresh surf clams for to use on the hook but maybe fresh clam bellies on hook would be ok, not sure. Maybe "chunking" fred will reply to this, he would know better than anybody probily.
Have to look at the entire clam. There is the loose meat like the muscle, strap and siphon. Nice to eat chopped up in chowders but not much flavor. Then you have the big tongue or foot of the clam. Great sliced, breaded and fried but again little flavor.
When they commercially bread the clam slices for fried clam strips they have to jack up the flavor. When they can chopped clams they have to jack up the flavor. Both are done with clam juice. The meat of a surf clam just doesn't have much flavor. That is what makes them so valuable. Most people like a slight flavor of clam in their chowders. They don't want to be overpowered by it. Restaurants can doctor the flavor by the amount of juice they add to the chowder. If they just kept adding meat they would come up with a very expensive bland tasting chowder.
The clam belly is what you don't want to see in your chowder. It is packed with the digestive organs that are green and brownish in color. Not what you want in a spoonfull of chowder. It is also the most overpowering area of the clam for both juice and aroma. It may not be what we want to eat or taste but it's what the fish want.
Take a bunker out for crab bait and whats the first part to be attacked by the crabs and fish? The stomach area. Throw a fish carcuss over the cleaning board and watch the minnows attack. It's the stomach area. It's where the flavor or scent comes from. With clams it's the same. The fish want the softest juiciest part of the clam first.
When I fish I chunk with the bellies and bait up with bellies. They always work for me.