Bay Area Cobra Club meets every Saturday For details
http://www.bayareacobraclub.com/pages/5/index.htm
The pictured Jag units mount on the top. The front lower "mount" is to prevent axle torque from rotating the diff around the axles. Some cars are mounted solidly to the frame in front, others have rods from the frame to the diff to prevent wrap-up. I second the T-bird diff if you need to bottom mount
One view of a t-bird mount
http://parts.factoryfive.com/roadster/chassis/irs.html