Why is my brake pedal so high ?!

two thoughts
1) I swear when I rebuilt my pedal box, that there was a small rubber stopper in the kit installed somewhere that caused the pedal to make a nice muffled thunking sound when it slapped UP against it, to it's Parked position.
2) if you have a period correct MOPAR manual MC, the pushrod is supposed to be held captive in the business end. Three things then determine the actuation of the MC;
1) the pushrod length, and
2) the depth of the pushrod-cup in the MC, relative to the firewall. and
3) the proper bleeding of the piston chambers.

If you do NOT have a period correct MOPAR MC, I have no experience with those.
If you have a MOPAR booster, the pushrod on the pedal side is fixed and you cannot mess with it. But the rod on the MC side still has to be set up to allow the compensating ports to work, same as the non-boosted version.

I only have experience with 67 to at least 75 A-bodies, OEM parts only, and no interchange problems here.
But, In my experience, properly set up;
>I have never seen a brake pedal as high as 10 inches, I think most will come in around 7 or less. And the gas pedal in a 67 to 75 A-body is without exception, always lower. The parking height of that pedal is determined by it's arm hitting the floor-bracket. If you bend it higher, you will have to change your underhood set-up, or block the pedal from over-travel, or risk bending the throttle arm when you floor it.
Any period-correct MOPAR MC can be used on any Period-correct booster with any Period-correct A-body pedal-box.

Having said that;
on A-bodies with a clutch pedal, the narrow-pad brake pedal is always lower than on the same car with no clutch pedal, and a wide-pad brake pedal.
and the clutch pedal always parks a lil higher than the brake.
Hope that helps

BTW
>on a manual brake car;
If you set your pushrod too long;
the compensating ports may become covered in the process. This will make it impossible to bleed the brakes. But if the brakes were previously bled ok, and the pedal is normal height and hard, then: over time, as the pad material rubs off, the pedal will continually get lower to the floor.
If you set the pushrod a lil too short, you just have extra travel, and maybe a lil rattle.
>On a booster equipped car;
the pushrod length will affect the timing and intensity of the assist, because of the way the control valve depends on the up-stream resistance. But if the pushrod is overly long, it is possible to break the control valve on a panic stop, and then the brakes will be nearly impossible to modulate, and the engine may end up sucking air from the passenger compartment.