What about the P31 Neo? I read that the Mugen 2 would fit into a P35 (+block one ram slot, which won't matter for me, as I only have 2 ram modules) , and the P31 has similar space in the cpu area. One thing that seems different are the left capacitors, they seem to be several mm closer to the cpu socket.
And if the Mugen 2 would not fit, any chance of the Mugen 1?
P31
http://img35.imageshack.us/img35/4545/p31h.jpg
P35
http://img38.imageshack.us/img38/5154/p35.jpg