Keywind,
Just wanted to say that I have a OWC sled used for mounting SSDs in a 2010 MacPro for sale on Ebay. Might be able to get it to you once the auction ends if need be.
https://www.ebay.com/itm/254310770071
Now for my story... Had a 2008 MacPro which I bought new in 2009. Decided to upgrade similar to what you've done. Bought a mid 2010 MacPro on ebay really decked out with ram, 12 cores, 3.33 ghz processors... a very fast machine. Then I bought the same gpu you have, two OWC sleds, 3 SSDs, and proceeded to migrate to Mojave. Sounds good except for the problems.
1) My Apogee Symphony did not work... had to go back to El Capitan on my 2008 to get that firmware update.. major hassle which I won't even get into.
2) My recording software, Digital Performer would not bounce to disk on the new computer... I mean it would stall making the machine pitifully slow to work on when I needed to bounce.
3) I spend hours and hours (lost about a month of recording time) trying to debug this until I finally returned the 2010 and got another similar 2010 thinking it must be the computer. But alas, my guess was wrong, and DP still was not working right.
4) Finally decided I'd be better off with a 2013 MacPro trashcan... at least I'd have access the next Mac OS because it was foretold that the 5.1 MacPros won't be able to upgrade after Mojave.
5) Got the trashcan and still DP not working, freezing during a simple bounce to disk.
6) Created a dual boot to allow me to boot to El Capitan and then do my bounces. It worked!!! Wish it didn't take since March 24th to July 7th to come up with this workaround.
So I sold my GPU and one of the sleds off the 2010 MacPro. My losses amount to quite a bit of time, effort, and money but I'm back and running again.
Note that Mojave can allow you to format a drive in APFS format, or Apple's new file system. If you don't format your boot with APFS, you can't get to the latest version of Mojave, 10.14.6
I've also had problems since getting the trashcan, sometimes when the computer wakes up from sleep, the screen flask or flicker making the computer quite useless. I hope 10.14.6 fixes this but yet to find out.