You cannot install 2 slot wide cards into a 780.

There are PHYSICAL Barriers that prevent the card from going in.

Radeon 3450 3470 4670 5450 6450 6670 7750 have single slot SHORT versions of the card.

R Series Cards DO NOT WORK.

If you are looking for a good and inexpensive upgrade get a GDDR5  GT1030 card.

Silent versions have no fan to fail and work in USFF Desktop or Tower

https://www.amazon.com/GIGABYTE-GeForce-GV-N1030SL-2GL-Silent-Profile/dp/B072HRD3CY/

ONE SLOT WIDE ONLY SHORT CARD ONLY

780 Series