There's no configuration of the converter brackets that makes it work right in the Mac Pro's apparatus. (Unless I am mistaken, but I'm pretty sure it doesn't work right.) Avoid! Big waste of money.
3.5 > 2.5 brackets will not work in any backplane setup unless they have been specifically designed to. It needs to hold the drive in a funny, off-center position so it lines up right. Any "backplane-ready" bracket should work. This isn't really news unless you are new to working with backplanes, but I'm sure it will be useful info for a lot of people.