MP 2,1 8 core memory riser in 3,1 2008 MP?

Hi,

I have a 3,1 2008 Mac Pro that is missing both it's ram sleds. I have found some risers for purchase from a 2,1 MP 8 core and wondered if they were compatible. I understand that most risers from 1,1/2,1 MPs are incompatible with my machine, but I have heard the 3ghz 8 core 2,1 is the exception to this rule. Does anyone know if this is true/ has tried it?