Update: new master cylinder came today.
Relieving the edges of the flange*:
*I grind the flanges to make room for the nuts that hold in the pedal cage, because dumbass me did not realize—and didn't think to ask if—you could simply clock the master cylinder 90 degrees to align with the holes in the common cast aluminum pedal frames you see everywhere. Instead, I used the stock horizontal holes Thunder Ranch bored in the front firewall to mount the MC, and then drilled new vertical holes for the pedal, which would work fine and not be a big deal if the pedal frame did not both block access to the MC bolts put through the horizontal holes and not fit unless installed first. This means that R&Ring the MC involves an intricate three-handed juggling act in which one first installs the MC loosely with its bolts, then installs the pedal assembly by wedging/sliding it under the washers on said bolts, then starts the bolts and nuts holding the pedal assembly in place, then tightens the bolts holding the MC using a stubby 13mm wrench to them one flat at a time, then tightens the pedal bolts, then loosens the MC bolts and the pedal bolts in order to nudge the pedal into its most central location, then re-tightens the MC bolts one goddamn flat at a time, then snugs up the pedal bolts, and then finally grabs a hickory rolling pin and clops oneself in the base of the skull because MY GOD! WTF am I doing with my life??