The manual and power brake "petals" are the same.
The master cylinder pushrod is in a different location for power brakes vs. manual brakes.
Power brakes add a multiplier to the pushrod, which also raises the pushrod (to booster) up about 1½”, or so.
’76 to ’77 (or maybe mid-year ’78) F/M-body will have a 4-bolt iron brake master cylinder. The lower 2 studs for manual brake master cylinder and lower 2 studs for brake booster share the same two lower firewall holes.
’78 (or mid-year ’78) to ’89 all use a 2-bolt aluminum master cylinder uses a different set of holes on the firewall.
I think that power brake multiplier is tossing a wrench into your calculations (maybe).
BudW