Forgive me as my experience with Dell computers is very limited, but would it not be possible to move the components into a different case?
Is it using the ATX form factor?
Are you experienced at building PC's?
If it does use the ATX form factor if should be fairly easily to find an old case potentially for free, many people/businesses will be happy to get rid of them, it saves them the trouble of scrapping them and this would enable you to install a wider range of GPU's.
UPDATE:
Another idea would be to buy a PCI-E 16x to 16x Riser like this one:
https://www.amazon.com/PCI-Riser-Ad...qid=1524119370&sr=8-3&keywords=pci-e+extender
Of course there are two problems with is, one would there be enough room for a full sized card, you would need to take your own measurements and the second how to secure the card inside the pc, you might need to get creative, this might involve lots of cable ties