Right i've been musing over this at work today, did some (very very) rough drawings and measureings.
I reckon, but dont quote me on it, The Diff is subjected to torque loadings on two axis - The longitudinal twist from the prop shaft/gearbox/engine. And the latitudinal twist from the road wheel friction (equal and opposite Blah de blah)
So the four main mount bolts hold the diff solidly to the rear beam which then opposes the torque of the prop shaft through the entire width of the beam and the butch mounts at each end (decent leverage, lower force)
The big ear at the back is a just a stabilising link. Friction from the tyre-road interface will try to oppose the driveing torque and force the rear of the diff off of it's centre line. It it didn't have the 'ear' it would be working on a lever about 18 inches to try and break the beam bushes. So they've thrown in this rear stabiliser.
So, Cut the ear off, solid mount the diff from two transverse box sections of of a pair off diaginal longitudinal box sections, job done.
I think they are M12's so get 8.8's and it 'should' be strong enough with the weight of a roadster
Cheers
Chris