The 4.2 muffler is designed to have the socket head screws recessed into the muffler down at the exhaust flange, is it not? You mentioned longer bolts and it looks like you installed them with washers on the outside of the muffler housing? The long bolt setup is not the answer. Think about you are unable to really tighten the flange as you are actually compressing the muffler housing which is welded to the flange.

I have six Taurus Engines and have never had a problem with the Taurus exhaust system. 2.6, 3.2's, 3.7's and a 5.7.

I don't use Loctite it's kinda self defeating, what breaks down thread locking adhesives, heat! The high temp adhesive may help but you'll need to note the working temps. I just don't feel it's required.

I don't use a gasket either the Ultra temp silicone in place of a gasket and metal strap Ralph mentioned is a must to secure the muffler. Apply a light film of the silicone to muffler and allow it to tach up, then install and snug the bolts very lightly. Allow the silicone to dry for an hour or so and then torque down the bolts. Warm up the engine, tighten the bolt's and do it again. Remember there are several dissimilar metals interacting here. Heat cycling will help allow things to settle in.

I also use internal serrated star washers. Like those that come with the metal gear servos on the SHCS.