If you’re not worried about total originality I would suggest removing the booster and the master cylinder, replacing the master with a unit from an earlier Alpine. The smaller bore gives a softer pedal pressure than the original master without the booster.
Don’t forget to rebuild all the masters, the clutch slave, calipers and wheel cylinders. The fluid is hygrscopic and corrodes the bores of everything. A friend just had his brakes redone and the rear cylinders were frozen. He had to buy new ones.
The March, 2008 issue of the Rootes Review has an article about redoing the brakes and front ball joints, using parts from Austin-Healeys and Jaguars where the Sunbeam parts were no longer available.
Fred Baum