[MrWumper]

Some interesting responses here...As for the differential "cracking" like an egg---that's exactly what I want to try and avoid in my design, although you may be fully correct nonetheless.

Chopper---I guess I don't see how the calipers would interfere with the frame rails (if that's what you were referring to) in this set up. If it makes any difference, the set up I'm using is from a '64 XKE (with a Salisbury differential), so it may be slightly different from the set up you that are looking at. The frame rails would be completely below the entire differential casing, and my calipers don't extend beyond this, but possibly I don't understand what you meant.

daltondavid---"What type of "CAR" do you have?"--- I was hoping it wouldn't be asked, because this IS a Cobra forum, however this seemed the only reasonable place for me to ask this type of question, especially since Jag rear ends are so extensively used in Cobra replicas. But, with hopes that the moderator won't delete this post for being off topic, the car is actually an (in progress) LS1 powered Triumph (TR4A---the original car already has an IRS, but it would never take that kind of power). I'd think my set up is actually much more closely related to a Cobra replica than to that of a Hot Rod (which of course also commonly use Jaguar IRS's), which is why I came here instead of a Hot Rod forum...

Anyway, I'm thinking of possibly having the frame rails cross under the diff. in an "X" type fashion (like an x-brace), intersecting directly under the center of the differential, and widening out from there (then boxing the center section under the diff.). Whether or not the appropriate angles are even possible for this using 3"x3" square tubing without interfering with the lower control arms (and without notching the rails at all) I am unsure of thus far.

Any support from the top of the differential would be extremely minimal, as having frame rails running across the top would necessitate cutting into the body, and should I do that, I might as well just mount it in the traditional fashion. It could bolt to the sheetmetal above, but since this is not a unitized structure (unlike the Jaguar), any excessive amount of forces feeding into this area would be trouble (i.e. should the lower mounts not be sufficiently rigid.) Virtually all of the torque would need to feed into these lower frame rails for this set up to be successful.

As far as the distance between mounting points, as mentioned by strictlypersonl, 4.2" seems to be the distance between the top mounts if I understand correctly. Or, do you mean the distance between the bolts holding the control arm mounts to the differential casing)? As for mounting the whole set up through the lower control arm brackets, the mounting bolts are 8.5 inches apart from front to back, and 9" apart side to side. But again, if you were referring to the distance between the bolts mounting the diff. to the control arm brackets, then you do make a valid point (and even if you were not referring to this, it is a valid point I hadn't yet considered.) Should those be too weak (or should they feed too much torque into too small an area of the differential casting itself), then the concept would certainly not work.

One more thing about this design is that it would need to use either shorter coil overs, or more steeply angled coil overs---of which either situation might potentially lead to an undesirable geometry (I'm unsure on this aspect). The shocks would mount to a frame rail above just as is generally done, however it would need to be lower than the top of the differential, unlike the standard set up, in order to keep the clearance low enough so as to not force cutting into the body.

Any further insight is much appreciated. I wish I could draw a picture of what I was saying and post it but hopefully you get the idea. From the gist of the responses thus far, I assume that no one here has ever seen this done before (and I'm trying to determine if that is for a valid reason.)