May I make a suggestion to see if it will work for you.

Take one of the old rubber bushings and cut the sleeve out of it.

Using a drill or I prefer an end mill, to open up the hole in the alternator housing and insert that bushing.

Even if you do not get a press fit (you can peen it, if you like) it will repair the hole with steel. This will now be of a larger diameter touching aluminum.

The larger steel diameter will spread the load by increasing it into larger internal surface area. The contact area with the aluminum will no longer get beat out into dust.

The strength of the bolt was not the issue here. It was tightness and alignment.

I did away with rubber bushings down there. I went solid PVC bar stock with fixed shoulders. I bored the bracket holes round, in-line straight and parallel with the base.
These brackets were sand casted instead of investment casted and are sloppy. I believe it's the reason for rubber. They slop or slapped together!
Not one of Volvo's finest minutes, in their hour of fame! IMHO

By the way almost all bolts use on accessories and are a grade five. You can use any bolt that has three or more radial bars on the head.
Regular old hardware bolts are a grade TWO. Come with No bars or smooth heads. They are considered to be made with butter metal in our mechanical trades.

The three bars and the background two, will make it a grade five. A grade eight will have six.
Grade nines come with paper trails and batch certificates and should have a dollars sign on them! (:)

Metric's use a number system that I'm not up to snuff on! I'm sure it's just as simple. Little Numbers like 10.6 They are not bolt or mm head sizes. You got to learn those like counting backwards.

FYI
Phil