Will an Older Mac Pro Power Supply Work with a Newer Early 2009 Mac Pro?

I am trying to build a Macintosh, (which I understand sounds insane), and I would like to get a less expensive PSU on eBay, rather than buying from one of those specialty Mac Parts stores. All I can find on eBay is the 661-4001 but the PSU for the newer Mac Pros is the 661-5011. They are the same wattage. Will it work?