The rear end is working great. I have had zero issues other than the rear end being a little loose in the turns. It is completely undetectable until you ask for it to be there. I have been beating on my car quite a bit trying to get the tuning in for e85.
I would really look at going with either the w129 case or the w220 case. The qdf8v is the same as the qdf7v except the gear ratios. It is simply a matter of finding out what gears it was set up for. But I would guess by the L4 dimension that it is a 3.27 or higher. This is a 210mm ring gear. You can run either one piece rear axles or bolt up flanges. The w208 clk came with bolt in flanges with this case as many others. If I snap an axle this is the direction I am heading.
It is tempting to just go for the 215mm ring gear setup though from a w209 because the qdf9v is so much cheaper. It would basically pay for the custom axle halves.