Using the red (or any) Loctite on the muffler bolts isn't a real good plan in my experience. To get red to release, generally you would warm it. Running it on any type of exhaust application will warm it the same way - and generally it will get plenty hot enough to release. Instead, consider cleaning up the threads the same way you would if using Loctite, but use a little of the same silicone you're using between the exhaust port and muffler on the bolt threads. Doesn't take a lot. If everything else is right, that generally works pretty good.
Flat mating surfaces is absolutely critical - both sides. If that muffler had a couple of tanks of gas run through it while loose, I can almost guarantee BOTH sides will need dressing.....