If the pressure fitting is comming loose on a muffler then it probably has bad threads on it, or most likley the threads in the aluminum of the muffler are gone. The problem with using ANY retaining compound is that they all go thin when exposed to heat. How do you think you get the stuff loose once it's cured anyway? Heat it up and it breaks it's bond pretty quickly.

It would seem to me that if a muffler fitting had a new peice of fuel line on it the fitting would be unlikley to turn in the threads in the first place, due to the fact that the entire tube would have to turn with the fitting to allow it to back out which is unlikley. If it were mine I would consider getting a new, larger pressure fitting and retap the threads to the larger size.