a locknut with locktite
despite any efforts to hold on your exhaust muffler bolts, double nutting them works best. install the first ones nice and tight install the other ones and while holding the first ones still with a wrench you tighten the second ones aginst them snug this locks them in place and i can assure you they wont move. you can double nut on anything, there is no crime...... unless your trying to bolt a horse. there are laws against such a thing.
anyway, teflon locknuts are best to use, since you never used any locktite before you are going to be looking for the blue stuff since it breaks loose with a wrench (the red type requires heat to loosen off, and green, is for bearings and bushings
as for the petcock, since its somthing that might as well be screwed in permantly i dont see why ou couldnt use a little seal-all on the threads as you screw it in.
hopefully that gets you started : )
you might want to go get youself a few inches of better fuel line too, that rubbery crap they call fuel line will harden up and start to leak within a month.