GX270 20 pin Mobo connector, ATX or not?

​Good day.​
​I have a question I hope someone can help out with. I have an Optiplex gx270 "slim Model" destop that is out of warantee. I plan to use it in a "carputer" application as it has always been fast and reliable for me. I will be using a DC-DC power supply (DSATX, OPUS, Etc) instead of the dell power supply. I need to know if the connector is ATX standard on the motherboard 20 pin PSU connector or if it is dell Proprietary in it's configuration. A pinout diagram would be great if one is available. There seems to be much conflicting information about whether or not the connector is ATX or dell proprietary. Once again, this is the slim line tower case. The connector on the PSU is ATX 20 pin but the PSU itself is a long flat PSU and doesn't look like the standard "square" PSU.The physical size of the power supplies I will be using is not a factor as the entire unit will be in a custom built case.​

​Some additional info that may be helpful:​
​Optiplex gx270 P4 2.26 GHz​
​Model# DHS​
​Mfg Date 092403​

​Thank you to anyone for any advice or assistance you can provide.​