If you can't hold bolts in it, stud out the holes in the block with threaded rod or long bolts that have been cut for their threads. Put heavy compression springs behind the nuts and washers.

The trick is don't try to keep the muffler tight. All you want to do is keep the muffler sealed. The right springs will do that for you. Hot two strokers, dirt bike and snowmobile exhausts been doing it for years, you'll even see it in production cars. You can't hold a steel screw in an aluminum block tight for very long if theres extreme heat and vibration. It has to do with coefficients of expansion and contraction of the two materials. Just heating and cooling it without even running it will loosen it, sort of speak

Hunt down the shortest, heaviest compression springs you can find and make studs for the block. You will save the threads in your block also before its past the point of it still having good threads to work with you having to keep putting a wrench to it. Thats only good for so long before things start to strip out and the frequency of having to tighten it goes up.

The pic i put up is a bolt-through motor with tapped header. I put RC Car clutch springs behind the screw heads instead of dealing with keeping screws tight in the header. The bolt threads go through the header, and a jam nut run down to keep it. An ape could not seperate the seal, yet no screws are holding it tight.