The old Nvidia Quadro Plex 2200d2 works with my 4,1/5,1 MacPro.
The 2200 version was made I think from 2008 to 2011, and its PCIE Host card, is PCIE 2.0 so matches the MP.
There is even a dual PCIE Host card that would allow you to connect two of them, and have 4 external GPU with one internal.
It gives two PCIE slots in the box with a 750watt PSU. The cables in it are 6pin + 8pin for each card.
After upgrading the cards in it to two K5000 Mac Editions, it has EFI boot screens too.
Since the Quadro Plex was never sold as a Mac item, it does have a few quirks relating to multiple monitors.
If you just want the Plex as a compute box, it works perfectly with MacPro 2009-2012.
It should be ok in slot 1 of a 2008 since its PCIE 2.0.
It works fine now with the new 10.13.4 and its EFI update.