I don't know why there is some resistance to cutting the flanges as you say, RoadrinninMark??
I have posted that solution maybe 10 times over the years, it works just as you say.
Never a leak here.....
Joe
same here. i always cut the flanges. plus, drill one bolt hole per flange oversize. this allows the flange to "float" a bit during heating and contracting.
no leaks on even the thin, flimsy flanges.
i stole this idea from a member here years and years ago, and it just flat out works !