Julian,
I think the difference in A-Arm length between the Si and earlier Panteras is about 4" and the front wheels have close to 60 mm offset, or more. The number 72 mm sticks in my mind! Simon can clarify; he knows exactly what the specs are!
What I found interesting was, the shock and anti-roll bar mounts appeared to be in the same place as they were on earlier Panteras. The extra length was all outboard of these mounting locations. Also, I'm not sure whether the Si has a different steering rack or DeTomaso used the earlier GT5-S rack with extensions for the tie rod ends.
Other than increased length of the A-Arms, the major change at the rear is elimination of the upper ball joint. The Si uses a shaft on the top of the rear upright, just like the one on the bottom.