You may want to consider drilling a smaller dia pilot hole before going in with the 8.5mm drill.
However a long series drill in a small dia might not be rigid enough.
And it might not centralise so well as the 8.5mm.
If you do fancy that, the same guy in the second listing has a 1/8 inch, long-series drill on offer but that might be too flexy.
Maybe a 4.8mm or 5mm drill from the first supplier might be a better bet.
I'll leave you to ponder that.
Note that although I speak as a bit of an engineer I don't guarantee that my suggestions will work.
But if it were my problem, that's the way I would be thinking of approaching it.
Finally, if it all goes wrong and you wreck the swingarm, I recall that Nasher was offering three ally swingarms for sale a while back and he may still have a couple left. I remember there being surprisingly little interest at the time.
http://www.ukmonster.co.uk/monster/s...ad.php?t=59456
Hope that helps, even if only to point you in a different direction.
And good luck.
ps.
As an aside, I wasn't aware when I fitted my ally swingarm that the steel channel/saddle washer existed.
My old steel arm didn't have one but obviously it wouldn't need one.
I can't really see the point of it anyway ... it only seems to hinder access to the nut with a ringspanner.
Maybe it was an assembly aid but I can't even fathom that tbh.
Anyway, I fitted mine just using a plain washer and I'd be inclined to recommend that you do the same.
The damn things are possibly no longer available anyway.
For what its worth, I also fitted a washer under the head of the bolt. Dunno whether that's std or not.