[MrWumper]

I know the Jag IRS unit was made to be top mounted (i.e. hung from the top of the differential as opposed to mounting to the bottom of the frame), but is it possible (or advisable) to "bottom" mount it, i.e. do away with any top mount and instead solidly mount the differential to frame rails below the diff (and mount it with bolts through the lower control arm brackets that originally bolt to the bottom plate)...?

Of course the rails would have to pass under the diff north-south as opposed to a top mounted set up of east-west (like the CWI set up, or any other Jag based derivative), which would set the frame rails very narrowly apart under the diff (to avoid interference with the lower control arms), but this set up would make it possible in my set up to not only not have to cut into the body on my car, but also keep the driveshaft angle correct by mounting it higher than would be possible by top mounting it (and thus doing away with a 2" frame rail above the diff.)

Every independent rear suspension I've seen, however, has mounted the diff to a kick up in the rear of the chassis, and top mounted to it. Is there a valid engineering reason for this? Would bottom mounting the differential give a weaker overall structure, and/or allow too much twist? Does a kick up in the frame with a top mounted differential put less stress on the frame, the body, or the differential, (or all three) or is it just done out of convenience or ease of design? Of course this would all make for a very "flat" chassis, but again, I don't know if that would be a problem in terms of strength or rigidity, or not. Any advice is much appreciated.