Thanks guys for your answers.
In fact, yesterday I decided to attack this issue with some of my tools. I was able to grind the welds and remove that Hex quick removal fitting.
I managed to do that without damaging to much the original shaft. The splines are still there and in 85% good shape.
The end threads are a little bit damaged though...so I'll think carefully about my next move.
And I understand the comments about this feature being practical to get in and out of the car. But since I'm 5'-7" and slim, I don't think I'll have that problem
And most important, I want to put back the original steering wheel.
So now I searching for correct replacement column switches to replace my old ones...Are they really supposed to be Triumph MK4 Spitfire ones?