The PSU is mounted next to the Car Stereo on the bottom shelve (you can see it in the photos of the boot). I did try to see if I could mount the psu in the M350 case, but I couldnt find anywhere I was happy with to mount it. I do have limited space in the case due to putting a dual-core atom in and having to mount a fan on the heatsink.
(see my photos of the case & mobo) It is possible that if you moved the HD mounting to over the side with the CPU and get the optional HD mounting, attach the PSU to that it may all fit.. however im not 100% sure I would 1.. want my HD over the CPU and 2 want the PSU inside the case.
Maplins to do a project box that the PSU will fit in, which I will be using when I build the shelves correctly.