Re: muffler gasket blowout
I fully agree about teh flanges not being true and I had to resurface mine on a sanding table before it was flat and true, then I painted the surface with blue machinist's dye and ran it over a sanding block made out of wood to check that it was perfectly flat.
I really don't like using bolts or allen screws where I'm torquing into the aluminum just as a rule of caution against stripping the threads from repeated torquing unless inserts or helicoils are being used. The aluminum can usually withstand a few re torquings but the softer metal tends to wear faster which is why I still prefer studs and nuts or some kind of insert like helicoils or kingserts.
The use of brass nuts on the studs also helps because the softer brass won't wear out the threads on the studs, it's easily drilled for safety wire holes and can withstand several re safety wiring. These also tend to stay put better when star washers are used if someone prefers not to safety wire them. I got a few sets of the brass ones that Fred sells in his ebay store and they're really nice because they are like 8mm wide so there's lots of thread surface to prevent stripping.
I've also did the double nut thing on mine before and that works really well to keep them from backing out. I do prefer to safety wire them when I can tho because if they ever do come loose they won't work their way all the way off and will hold everything together long enough to get home.
Another thing that causes the exhaust bolts or nuts to constantly work their way loose is when the pipe is installed without proper support from extra mounting brackets, the brackets that come in the engine kits are just thin metal and they can flex enough to let the pipe flex with the engine's vibration, this in turn wears out teh gaskets quickly and the nuts become loose enough to back off and fall out so I also strongly recommend making your own brackets out of thicker stronger metal that won't let the end of the pipe flex from engine vibes or a bumpy road etc...