Nick
The advice above is no doubt accurate but there are plenty of Victorian cars that have had this conversion over a long time and I've not heard of any shock/suspension system failures.
It also has to be remembered that the cobra carries far less weight than the original Jaguar, perhaps this is a mitigating factor.
Anyway, my car's journey to date and I am happy with the handling and engineering.
Single adjustable shock with 400lb spring to rear of control arm. Original springs were 250lbs each.
Heavier drive shaft, double rose jointed training arm and adjustable lower hub to control arm adjustment. This combination made rear end alignment a dream
Inboard disc conversion to XJ308 outboard disc amendment with larger piston option. The car has significantly increased braking capacity as a result and no access issues to the rear brakes the inboards pose.
Anyway if you have build time, it would be worth a chat through the Victorian club on this option.
Cheers