I would think actually since the A2000 PSU is structural - helps hold the case together - you'd need to use the backplate of an A2000 PSU to line up where to put mounting holes in the ATX PSU.
Wonder if the ATX and A2000 PSU's are even the same width? My A2000 is actually closed up for once, but next time I open it I'll check. Was thinking if I ever went down this route I'd try to find some kind of small MATX or Pico PSU, so I'd know there wouldn't be space issues, and then fab up some kind of scrap metal plate to hold the back of the case together.
If you do this, please take lots of photos & document your process! :idea: