To be on the safe side the maximum GPU length you can use on OptiPlex 3020, 7020, and 9020 is between 9.5 and 10.0 inches.  190 mm long card should be okay. you can upgrade psu to any 24/4 pin ATX psu plus 24 pin to 8 pin adapter from Walmart/Amazon etc.  No need to install psu outside mt case.  
Hello everyone. I've heard on the device that there are only 8-pin connectors for PSU's however there are 8-pin to 24-pin connectors however I am unsure if this can fit in the MT model. Can I wire it outside of the device? Also, could the MT model hold a RX 6600/RX 6600XT? It's dimensions are 190 mm x 110 mm x 40 mm. Thanks!
Can I upgrade the PSU on the Optiplex 9020 MT for a RX 6600XT