Welcome to the forums. I think we need to straighten out which model card you have before finding the proper block. The 3958 card shows to be a 3080ti XC3 Hybrid. Is that what you have to begin with? If so you need the XC3 block not the FTW, they have different layouts. You could try the Corsair one or EK makes one. 
 
https://www.evga.com/products/product.aspx?pn=12G-P5-3958-KR&associatecode=LMD3DNZM9LGK8GJ
^^ Is this your card? 
 
https://www.ekwb.com/shop/ek-quantum-vector-xc3-rtx-3080-3090-d-rgb-nickel-plexi
^^ XC3 block
 
https://www.ekwb.com/shop/ek-quantum-vector-xc3-rtx-3080-3090-backplate-nickel
^^ EK makes a standard backplate. 
 
https://www.ekwb.com/shop/ek-quantum-vector-xc3-rtx-3080-3090-active-backplate-d-rgb-plexi
^^ EK offers an active cooling backplate for the cards. The cost is about like buying another block for back. 
 
https://www.ekwb.com/configurator/waterblock/3831109832646
^^ Shows compatible with 3958, and other XC3 cards. 
 
https://www.amazon.com/EKWB-EK-Quantum-Vector-Nickel-Acetal/dp/B08N7WZPY2
^^ EK makes different models {Acetal, plexi, ect.} 
 
I have nothing against Corsair, just never used them for Water. 
Compatibility shows nothing available for XC3.  
https://www.corsair.com/custom-cooling-configurator/gpu-search
 ^^ 12G-P5-3958-KR
 
https://www.performance-pcs.com/water-cooling/water-blocks/water-blocks-vga.html?block_gpu_type=426&vga_series=3316
A few choices. 
 
 
Hope that helps some.  

What the others shared is something you need to post so others can help. 
 
It appears that your gpu was a hybrid and still has the factory copper plate in place that helps transfer heat from the components to the AIO block cold plate.  That copper plate is not present usually in the air cooled cards I've seen.  I haven't taken apart a hybrid yet, but it looks like the plate in my Kingpin which you will need to remove eventually to mount a block on it.  Your block more than likely is not going to work with your gpu if you bought the block for the wrong gpu.  You might be able to sell it, but a little more research before you buy parts can go a long way to make sure you don't waste money.  If you are not sure, you can post questions here or on other forums and members will help you.  If you haven't done these things before, a youtube search can help a lot.  Knowing what others have done and options you may have will help you in the process.  
 
 

OP, I'm only going to reiterate what others have said. The copper block that is specifically part of the hybrid kit that was preinstalled on your card is not going to be compatible with any GPU waterblock. It wouldn't even work with EVGAs Hydro Copper waterblock.
 
That big chunk of metal doesn't cool your memory or VRMs. It transfers heat. When you are transferring heat to water that is continuously flowing you don't need or even want that extra hunk of metal, you want the metal that is integrated in the waterblock. If you did what you seem to want to do you would have mem/VRM --> thermal pads --> big chunk of copper --> thermal pads --> waterblock which would perform FAR FAR worse than mem/VRM --> thermal pads --> waterblock.
 
The goal is to transfer the heat from the component to something else that will carry it away. Either air or in this case you're going with water. Thermal pads and chunks of metal are much better than an air gap, but they aren't as good as getting that heat to the water faster and with less interference. The hybrid kit cools the mem/VRMs with air, so it is designed with that in mind. 

Yes you gave to strip the card down to use a waterblock and clean the card very well