Use spacers between the flanges over long bolts
I also built and installed a Superior O-360, but I used the stock oil cooler. I believe the key to not breaking the flanges is to use spacers between the flanges over long bolts that go thru both flanges. Therefore, the flanges are not being compressed, rather, they are just used to locate.
I know there are pics of this on Doug's website, but if you cannot find them I will be happy to take pics of my installation when I next remove the cowl.
141 hours thus far, no problems with the cooler.
Barry
Tucson