69 Barracuda Formula S with front stock discs, A199-1 SSBC calipers upgrade disappointment

Yes I have heard that, here of FABO, a few times
Since you are in France<
You might try the biggest M/C in your parts bag, and then re-engineer your Pedal-ratio. Looking at those calipers, I would suspect the calipers are flexing due to insufficient mass.
BTW
I have a factory KH 4-piston kit on my 68 Barracuda with a 7/8 M/C and on the street it's never let me down. But on the back I have a 10 x 2 drum kit running unproportioned with 295/50-15 BFG T/As. That rear system pulls down hard.