Might I also suggest, unless you are really set on that particular rack, that you use another one designed for braze-ons? The rack you have here was designed either for the sandwich method of attachment or, ideally, for being bolted to a "Pletsher-Plate" brazed onto the frame, and it really relies on that for lateral stability.
https://de.wikipedia.org/wiki/Pletscherplatte
Part of the charm and flair of this rack, to me at least, is the somewhat awkward, utilitarian mounting. Hacking a way to make it fit to braze-ons contradicts the reason that rack exists in the first place. I like Pletscher racks, don't get me wrong, and have used them on several bikes with good results, but have stuck with the ones that are designed for either brake bolt mounting or braze-ons
I also checked, the new version of your rack, the CS, only has a single bolt in the middle, so the new version isn't compatible with the adapter I mentioned. I would guess this means yours probably isn't either.