A x16 card will work in an x8 slot, but will be limited to x8 speeds...
 
Looking at that board, that's PCIe 2.0 x8 slots as well. I'd think a 2060 (or 2) would be the most you'd wanna put into it as it's going to bottleneck.

They are not open-ended and I am really not wanting to mod them if i don't have to, hence why I figured I could attempt to find some 8x cards...I know I could mod the slots, but I am trying to keep that as an absolute last resort type thing if that makes sense.

Opolis
Obviously you wont be gaming with this configuration but the performance hit should be small for folding and the alternative is not being able to run the GPU at all or modding the slot.
You could get/make a mining style GPU rack and mount outside the case.
 

This is probably your best bet with that setup... then get some low power 1660s or the 2060KOs in there.

They have PCIE x8 to x16 risers.
They do rise the GPU by about 0.75", which works in case your motherboard is horizontal, and the GPUs are vertical.
You'll have to remove the front mounting lip (it won't mount in your case), but can rest safely on the riser.
 
I wouldn't go with USB risers, as they are limited to x1 speeds.
 
You also can look into your bios.
Chances are that your slot isn't serving x8 speeds, but lower, due to other GPUs occupying PCIE lanes.
In that case, you can use a PCIE x4 ribbon riser, or x1 risers as well.
Anything x8 or lower needs to be powered.

Limited space is always a problem.
Are the GPUs half size?
Does your server only accepts blower type, or also open type GPUs?
Also what's the max slot width for the GPU? (1, 2, 2.5, or 3 slots)?
Fourth, can they even get cooling inside the case? PCIE 8x without pcie 6/8 pin cables, could mean you're limited to about 50-60 Watts GPU max.
That would mean very old and slow GPUs.
Aside from cutting open the back of the slot, you can also cut a slit on a pcie x16 GPU on the pcie Lanes, to make it fit a closed x8 slot.

There are a lot of factors to take into consideration in that design.
My personal opinion, is, the highest compatibility probably lies with a Quadro-P series GPU on this server.
Check your BIOS revision date. It probably won't be able to support GPUs beyond it's revision date.

I've looked at the specs of this server, and it doesn't even list any compatible GPUs at all.
More than likely, GTX 1600 series or newer, and RTX Quadro GPUs aren't supported, because they're probably going to be newer than your BIOS will be.
If you do want to run GPUs, because of the design; unless you run the case open, or cut/drill out a hole where fans are running on an open cooler GPU, open cooler designs are out of the question.
Meaning you'll be stuck with a blower type GPU.
 
Luckily most Quadros P-series are blower type. So even here Quadros are preferred.
Or, a higher end Quadro K-series, like a Quadro K6000.


 
The x8 slots are PCIE 2.0, and for network or SSD adapters. Not sure if the 8 lanes are shared, or if it's x8 per PCIE slot.
Since it's PCIE 2.0 at x8 speeds max, any GPUs faster than a Quadro P6000 (~RTX 2070) are going to be PCIE bottlenecked.
If it's x4, you'll be looking for a Quadro P2000 (equivalent of a GTX 1650). 
I wouldn't go below Quadro K6000, M4000, or P2000. Not worth it.

Anywhere between a P2000 and P6000 on the second hand websites, like Ebay; where they go from $100-600 second hand depending on the model.
Just don't buy the 'Quadro 2000', it is MUCH slower than the 'Quadro P2000'.
 
I would recommend you to purchase either a few items of these 90 degrees risers:

 
Or these, that are a little taller (whichever would fit):

 
 
 
Buy a couple, and use a grinder, saw, or steel file, to file off a little cut so the x16 slot would fit in an x8 slot.
It's a better solution than modifying the x8 slot on your server, or modifying your GPU.