I had luck. I was cornering, but with no speed, so I just finish on the side in the grass, without too much damage. Nothing to the body itself or the floor of the body.

The trunions had never been changed, and the car was equiped with very large tires (180/60/13). I know it's too big but I bought the car with it three years ago and never thought it could generate such a problem. Furthermore, the trunnions had been lubricated with normal grease, not with gearbox oil as specified.
I think these two factors are responsible for this impressive break, happily with no consequence. It's all fixed up now, and I found a new Spitfire, easier to drive with its normal 155mmm width tires !