Nah, the opportunity to get the Dremel involved rules out masochistic motivation. But what bit did you use? Carbide 1/8" or a stone? I'm having a hard time receiving the image of the hardened bolt and hammer use.

So you had to get longer bolts, right? Using a tap was how I started (because then, the bolts shipped with it would be long enough), but I quickly found that unworkable unless I could get the flange to a vise, and not just my hand-stabilized Vise-Grip pliers.

The existing holes in the new downpipe seemed appropriate for the M10 tap. I just knew I'd not get the tap straight (perpendicular) without getting the flange clamped in something solid and unmoving so I could use both hands on the tap handle.

In the end, it seems this monkey business around "genuine parts" would not have been tolerated in the Volvo dealer shops during the warranty era. Unless someone can say for sure the "blue box" down pipe has the appropriate flange along with its male outlet (needing no donut).
--
Art Benstein near Baltimore

What engineers say and what they mean by it:
"Developed after years of intensive research"
It was discovered by accident.