Question Would a gtx 980 ti fit in a 7020 MT optiplex and be able to function properly

This article should be helpful:
Sounds like a stronger ATX psu and a 24 pin to 8 pin adapter cable like this:
https://www.amazon.com/Eyeboot-8-Pi...aaf90a9138ce88e10aa210f5b783b5&language=en_US
For a GTX980ti, you are looking at a 650w or stronger psu.
DO NOT buy a cheap psu.
Look for one with at least a 5 or 7 year warranty

If you now have a GTX980ti measure it and see if the length width or height gives you problems.
Likely you are ok there.