So the last parts needed for this upgrade surprisingly arrived yesterday and I've just completed this mod. Unlike the OP, I didn't use a sleeve for my antenna cables since I think it is unnecessary and you can very well manage your cables without it. I also did my Bluetooth cable in a different manner. In my case it wasn't necessary to cut anything since I had depinned the original Molex connector before and just joined cables by using another pair of two similar short cables and twisting them together (also adding insulation between them of course). Here are some pictures of the upgrade:
I also added an NVME adapter card to the antenna bracket since I still wanted to make good use of the upper PCIe slot:
Another view of the logic board mounted inside the case:
A picture of how the Mac Pro now looks seen from the rear:
All in all, I have to admit that this setup works way better than with the internal antennas. In the process of taking out the motherboard I also removed the internal antennas (i.e. the ones that sit behind the sticker on the bottom of the Mac Pro and the Bluetooth one that was located near the bottom PCIe slot). I've tested a bit and everything works like a charm: Bluetooth, WiFi, Handoff and AirDrop
I was amazed by how easy it actually was to fully disassemble the cMP. I'd suggest everyone to do the same mod if they have every toyed with idea of adding Bluetooth 4 and other goodies with external antennas to the Mac Pro.