after cutting the flanges, i always enlarge one bolt hole two or three sizes on each flange.
that allows the flange to move around some during the heat/cool process.
i also use studs instead of bolts. if the nut interferes with the pipe, [unless the pipe is clearanced for the bolt] i use a flange nut with a reduced nut size. flange nuts also come with locking serrations if you want that feature.
i have been doing these things for years, having "stolen" the idea from member Mr. P-Body, and they allow even the thinnest flanges to stay sealed.
remflex [? spelling] gaskets are the best to use in all applications !