Add sausage links; reduce heat to medium and simmer for 10 minutes.
Remove links from broth; reserve the broth.
Remove casings from the sausage.
Bring the broth back to a simmer and add the quahogs; cook until they open, 5 to 10 minutes.
Remove the quahogs; reserve the broth.
Remove the cooked quahogs from the shells. Separate the shell halves. If necessary wash the shells.
Place the sausage and quahog meat into the bowl of a food processor; process until chopped, about 12 seconds, depending on your processor. Scrape mixture into a bowl.
Add chopped onion to the processor; chop about 5 seconds. Stir in to the meat mixture.
Make the full container of stuffing according to package directions, using the margarine, and substituting the sausage/clam broth for water. There may be more broth than you need.
Mix together the stuffing and sausage/clam/onion mixture. Spoon filling into empty clam shell halves and top each with a small pat of butter (about a third of a teaspoon).
Place the shells on a baking pan; bake in the preheated oven until toasty brown on top, 15 to 20 minutes.