The PSU is proprietary, making your quest more difficult. You may be able to source a 750 watt or 1000 watt PSU from the Aurora R13 or Aurora R14 . . . which I believe are the same proprietary design. I am not sure what PSU wattage was offered for the G5 Series, but I think that is also the same proprietary design.

The R13 (and XPS 8950) 750/1000w psu has 10 pin Dell proprietary motherboard white connector+ 4+4 pin cpu connector.   (460w psu has 8+4+4 pin)
XPS 8940 (and G5 5090) psu has Dell proprietary 6+4+4 pin.  The XPS 8940 6 pin is not compatible with R13 10 pin (or 8 pin).

the max oem psu Dell offers for 8940 is 500w.  FAQ XPS 8940
To upgrade 8940 psu above Dell oem 500w, options include aftermarket SFX 750w or higher psu that is small enough to fit in 8940 case and 24 to 6 pin adapter from Amazon.  This has been validated by at least one user.

for example, silverstone 750 w SFX dimensions 125 mm (W) x 63.5 mm (H) x 100 mm (D).  cooler master sfx 750 (125mm x 100mm x 63.5mm). both would fit inside XPS8940 of dimensions: height. 367 mm; Width. 169 mm; Depth. 308 mm.

drawback:

• cannot screw mount SFX psu without modding case
• the rear panel of 8940 blocks some air vent of SFX psu
• the PSU stays on after shutdown and requires user to flip the switch for it to turn off.
• cable management is bad

Photo credit: onefunkydude

adapter COMeap 24 Pin to 6 Pin ATX PSU Power Adapter Cable for Dell Motherboard with 6 Pin Port 13.3-inch(34cm) on amazon and it works fine. https://www.amazon.com/gp/product/B08M5BZ6P8/ref=ppx_yo_dt_b_asin_title_o02_s00?ie=UTF8&psc=1.

Glad I found this thread.  I am looking to upgrade the PSU on my XPS 8940.  Thanks to the above I know I need an SFX PSU.  

My question is can any share some insight into how you modified the case back panel to mount the SFX PSU (specifically for the screws) and what you did to ensure the SFX PSU had sufficient vent holes?

Thanks in advance for your help!

Happy New Years!