RE: Building a new computer inside of an Optiplex GX280 Case
If you can fit an mATX board to the system tray, you should be able to adapt the front panel wiring for standard Molex connections. The front audio is SoundBlaster-type and the pin assignments have been published, but the front USB port, HD activity LED, power switch and power LED are proprietary and you would have to trace them. I suggest using a continuity tester for that purpose.
Front-X sells Molex pins and plugs. The pins are intended to be crimped on with special pliers (one for the strain reliefs and another for the actual electrical connections), but you may be able to manually crimp the strain reliefs and then solder the wires to the pins.
You might be able to fit this Cooler Master power supply with little or no modification of the chassis back, which doesn't have provisions for a power switch. There were once a few power supplies without power switches, but I haven't seen any in some time.