Which way do the screws go? Into threaded holes in the engine, or go into the back of the engine and thread into the muffler? If it is the latter, make the muffler full width so you can make make as big a chamber as possible. On the ling ne in the picture, you can put 4 rows of holes if you need to . You can always plug them with JB-weld if you don't want to tap the holes. And engine HAS to exhaust if it is going to intake air. If you restrict the exhaust too much, you won't intake much fuel and air mixture. It's the "banana in the tail pipe" syndrome!!
Type at you later,
Dan McEntee
PS. If you can find the right tubing, and know some one with a TIG welder and some skill with it, you can make a tube muffler using what you have as the basis.