Which SSD for a 2010 Mac Pro and how to configure the system

The approach suggested by FatMac>MacPro will indeed work and is probably the simplest. A slight variation on it would be to get this instead NewerTech AdaptaDrive 2.5" to 3.5" Drive Converter Bracket

However these approaches only give you the ability to use a SATA SSD and worse only at SATA II speed. The later is because the built-in internal SATA drive bays in the Mac Pro are only SATA II being such an old computer.

A faster solution would be the Apricorn Velocity Solo x2 - Velocity Solo x2 - Extreme Performance SSD Upgrade Kit for Desktop PCs and MacPro® this uses a PCIe card and gives you SATA III which is twice as fast as SATA II. This would give you a real world speed boost from around 250MBps to about 450MBps.

An even faster solution would be to get a different type of PCIe adapter like this one - http://www.lycom.com.tw/DT-120.htm and then instead of using a SATA SSD you would use a PCIe SSD like this Samsung AHCI M2 SM951. See http://barefeats.com/hard200.html

As you can see this gives you nearly 1500MBps!

All the above are bootable and work with whatever Mac operating system your model of Mac Pro supports. In the case of the 2010 this includes High Sierra. (Not yet sure about APFS support.)

First, I'd suggest using this drive sled for the SSD installation. Then I'd suggest checking these SSD's out for the size you want. With the SSD in an adjacent bay, you can clone your current system to it and be amazed at the increase in speed. Finally, since High Sierra makes fundamental file system changes on an SSD, your applications might have some difficulty interacting with it.