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I have worked on Porsche transmissions for many years and am fairly certain transmission flanges were made using the case hardening process. Case hardening produces a hard surface usually about 0.5- 1.0mm thick however this can vary depending on the process used. I have never done the work you want to do but I have had case hardened 356 mainshafts machined to reduce the diameter from 20mm to 18.5mm where they pass over the differential. I have watched a retired master toolmaker do the work and he had no problems but his cutting tool was best quality, correctly sharpened with correct cutting speed. I think you should be able drill out the roll pin holes and cut threads with very good quaility tools. You might face off the blind holes on the back of the flange with a grinder which may get you through the surface hardening. You might look for a junk flange to practice on.
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