I got the Mercury 6G, kit and tools from OWC. If you've done this before you can see from the pix that I ordered the wrong data doubler. Here is what I should have purchased for mounting, though I sort of McGuyvered it and it seems solid enough (i don't plan on dropping my iMac). I would not recommend that as an approach, but I'm generally hard pressed for time (and impatient) so I'll live with it.
I've never cracked an iMac before... its impressive how they pack a computer in there and its hard to beleive thats considered roomy compared to the latest gen.