In agreement with @donm4599x.  Dell is too proprietary down to front panel wiring, fan wiring, MB screw holes, and layouts of the rear panel don't just vary, but vary in between different Dell models.

Just so you know:

Your processor is too weak for this graphic card on 1080p resolution:

Intel Core i5-2400 (Clock speed at 100%) with NVIDIA GeForce GTX 1650 (Clock speed at 100%) x1 will produce 63.6% of bottleneck on 1080p resolution. Everything over 10% is considered as bottleneck. - PC Builds

A couple reads about other CPU's for the 390:  https://www.dell.com/community/Optiplex-Desktops/390-CPU-upgrade/td-p/6083830 

https://www.dell.com/community/Optiplex-Desktops/390-CPU-upgrades/td-p/5975479 

I Googled "optiplex 390 max processor" and that's just a couple of things that came up.  Though the I5-2400 is the max listed CPU, some users have tried others.  The I7-2600k looks like a no-no, and Optiplex BIOS doesn't have OC option anyway.

Agree or not I installed the 1650 in the 390 for what I needed it for works great no more frames dropping on movies, it may not be the right set up for gaming but since I do not do any gaming big deal.

Nothing wrong with it if it works for you.