First question to ask: 

• Is this a proprietary Dell PSU so another one won't fit the available space or won't align with the mounting holes?

Second question to ask:

• The  Quadro 5000 graphics card alone draws 157 Watts and needs an auxiliary power connection to the PSU. So can you find a smaller PSU that will feed the GPU and still have enough connectors and power left over for the rest of the system needs, even if you only have one CPU?

I don't have answers to either of those questions...