what ssd will work in Mac Pro

As far as I can see the Samsung SSD 960 Pro is a NVMe type SSD and not an AHCI type SSD. This is relevant and important because NVMe is a newer technology which was designed specifically for SSDs whereas AHCI was original designed for USB drives i.e. hard drives. Unfortunately your Mac Pro pre-dates the introduction of NVMe and therefore does not know how to boot from an NVMe drive.

In fact until High Sierra you could not even use it as a data drive in your Mac Pro let alone a boot drive.

So, if you are willing to only use it as a fast secondary drive i.e. data drive then it could be used, but if you want a fast boot drive it is not suitable.

Samsung do make similar SSD drives to the 960 Pro which do support AHCI, e.g. the 951 Pro. (Be careful there is a 951-NVMe Pro as well.)

Sorry -- not the first time I've done that! They do have options for the Mac Pro as well but I don't know enough about them have an opinion. OWC SSD Upgrade Kits For Mac Pro Cylinder 2013

I don't know about the Samsung but the following source has a good reputation: SSD Upgrade Kits for MacBook Pro Retina Display 2013-2015