Emmanuel,
Someone has already pointed you in the direction of Andy Shepherd. He runs the register and will be able to give you a good steer. From memory survival rate is pretty good - in one form or another.
But a small aside on the oft published production figures, for I believe there to have been 31 convertibles and 50 fastbacks produced, not the 29/51 quoted by all.
Why? In the late 70s I ran CF17, a manual convertible and CF 19, an auto fastback. There wasn't much written about the 428 then so I did a bit of my own research and ran my own register. First point of call was the factory. They told me 29/51. I still have the letter, and so perhaps I am responsible for these figures being set down in stone.
However, in corrspondence with other owners whenever I heard of a car I learnt that one of the cars ACs listed as a fastback was in fact a convertible. This was a CFX car exported to the USA. So 30/50.
The 31st convertible is CF 81. This was to be a 'mkll' version of the 428, identical in spec to the ones that went before, save for fold down headlights. The car lay in the factory unfinished for some years before being completed in the early/mid 80s. It still survives, and is not to be confused with the 4 seater prototype which was a totally different car. Hence 31/50.
Change your auto box for manual? I wouldn't bother personally. That 428 suits the auto box so well. My party trick with CF 17 (manual)was to start off in top gear, which is a measure of just how torquey the motor is. And the auto was just much nicer to drive.
Hope this might be of interest.
Tim.
