Hey Crazy Jerry,
Good choice on the Quantum Mechanics differential. In my opinion the 3.27 is the way to go for the fuel economy. The later GT6 3.27 differentials were more robust than the MK1,MKII and spitfire units. Most likely this is the case they sale. As far as internals they are most likely a hybrid as the GT6 gears were actually not the strongest!
The major problem with a GT6 and Spitfire differential is failure of the crown wheel carrier due to fatigue. All Mk I and II diffs suffer like this and non overdrive Mk IIIs. (3.89 Mk III diffs are much better and less likely to fail). A car that is driven hard will tend to suffer earlier but there is also an element of luck involved. When the diff is out of the car the rear case can be removed and the carrier checkd for cracks very carefully just under the crownwheel teeth. The longer the cracks the shorter the life particularly if even more torque is put through it.
There is a hulk version of the 3.27 diff carrier (part number RKC2802) that is virtually indestructable, but it was only fitted from around 1977 and never to a GT6 go figure. You could find it on the Ital 2 litre auto, Swiss Spitfires, Dolomite 1850 Auto, and TR7 auto with four speed axle.
Most people today would look down on a vehicle that does not have power steering or power brakes. I find it to be more efficient and a huge reason to use the Spitfire/GT6 chassis as a kitcar design or super efficient one off dream car build. It is hard to beat simplicity and the original Spitfire exemplifies it.
I really like your side mirror selection now can those flip inwards also by chance?
Good talking with you Crazy Jerry and thanks for sharing more details of your Centurion. I am really enjoying our discussion here and this thread
GH