If a new booster doesn't solve the problem like it has for some folks -
You could consider putting a manual-brake pedal on with the vacuum booster (and evaluate as prudent).
I did this to my 65 Fury years ago by mistake, before I knew that the pedals differed between MB and PB. (by years ago, I mean about 30 of them)
I converted the car to disc brakes, installed the disk-brake booster (all parts from a 71 C-body), had to modify the '65 pedal bracketry a little for the nose of the booster, but otherwise everything fell together like it belonged. Modifying the brake pedal seemed par-for-the-course as the parts were 6 years apart, Fusey vs Slab.
When finished, the brake pedal always felt soft. (I'll spare all the diagnostic checks and all the 'experts' who couldn't solve the problem)
But the car stopped really well, better than our then-new 91 Buick, and on par with our 91 Camaro.
The pedal has the longer travel of the manual brakes and reduced effort of power brakes.
Might it give the desired result here? You would need to decide for yourself, though.
It definitely would need review for whether it over-strokes the booster, but that might be a simple geometry calculation between the 2 pedal setups. (i.e. do they have near-equal pushrod travel?)
My hunch is that it might not matter, as the MC piston travel (and therefore pedal travel) depends on when the hydraulic pressure causes the brake parts to bottom out at the wheels.
As I think about it, I suppose one might be able to over-pressurize the hydraulics due to extra mechanical leverage with the vacuum boost, but I suspect that SAE engineering principles dictated for safety factors in the seals, hose construction, etc, to be far more robust than the hydraulic pressure in the system. Or does the booster simply add a constant amount of additional force, independent of the force from the brake pedal?
And regardless - in normal driving, you won't be pushing the pedal anywhere near as hard as in a factory MB or PB setup, and obviously you'll adjust your leg to the required braking.
So it would boil down to - if you pushed the brake pedal as hard as physically possible, harder than you'd ever need to, like you were trying to break something - what happens?
Does somethign break? Hose burst? Wheel cylinder cups blow out? Does the MC piston merely bottom out in the bore? Or nothing unusual happens?
One advantage in this setup over mine - it is using the drum-brake booster, which gives less boost than the diskbrake booster in mine, so less force available to break something.
