I remember this nasty surprise. I think the earliest blocks were threaded but Volvo stopped bothering at some point.
I indeed tapped mine. The biggest challenge is getting it square. When I did mine, I was a dumb kid and expediency often trumped... doing things properly.
The right way to do it would be to use a milling machine so things stay square, I did it by hand, as kids do.
It wasn't quite square, but I was at least smart enough to know it was not square. I knew that getting oil filters to seal would be a pita so I got a remote oil filter kit. I loaded the o ring that seals to the block with black Toyota goo and put it on there good and tight.
It never leaked, but it in hindsight it was not "proper". Lol