Quahogs would be perfect if you can get them. If not, then try to find the biggest clams you can. But you need to make sure the meat of the shell is cooked prior to stuffing. And you can use a prepackaged stuffing mix if you care to.
A couple of "absolutely you musts!"
Scrub the shells prior to steaming and opening them.
Steam them first, then when the shells open remove them gently trying to save the juice inside them. That is FLAVOR!
Cut the meat (animal) from the inside of the shell over a bowl while trying to catch all the juices in the bowl.
Strain the water from the steaming with the liquid in the bowl through a cheesecloth. More Flavor!
Chop the meat into bite size.
Mix the chopped meat and the saved liquid with your chosen stuffing. (I would suggest Stove Top as it has to be cooked prior to serving. I don't know what is required with other commercial stuffing mixes. Have never used them, have always made mine from scratch. But the stuffing should be cooked prior to mixing with the cooked meat.
If the stuffing mix seems to be too dry, then add whatever liquid the package calls for. Mix the stuffing mix when ready and stuff your shells with a heaping pile. Pack it down. Place under the broiler until the top has a nice brown crust.
Additional notes: "Better Than Bullion" has a flavor for clams. You can use it to enhance the liquid you add to your stuffing. Just dissolve it first in the broth before you add it to the stuffing. You want to try to capture as much of that sea flavor as possible.