The firewall flexes like buggery, have a glamorous assistant jump up and down on the brake pedal whilst you watch the firewall flex.
Somewhere on here there are drawings for a brace to stiffen the mounting point of the brake booster/master cylinder assembly.
Also check to state of the flexible brake hoses, there are stainless braided hoses available.
There are larger brakes available too.
All it takes is money đź’°
"Keep calm, relax, focus on the problem & PULL THE BLOODY TRIGGER"