I know from previous posts manual and power pedals on these mid 60's C bodies are different but why is that exactly?
It's all about different leverage points between power and manual brakes. In the pic below the LH red line is the pivot point, the RH red line goes to the MC pushrod. For PBrakes, the pivot point would be raised near the blue line, to reduce the leverage. (that is sometimes hard to visualize)
Of the Fusey pedals I pasted earlier, I cannot tell in that pic if for PB or MB.
Anyone know if this is specific to the mid 60's C bodies or was it true for others?
I don't know all the car lines, but I've not really seen mention of it in other camps. I do know there was a MB->PB field retrofit for Cs, and it involved some lever-bracketry underhood. I believe A-bodies of the 70s had a booster moutned on some special brackets, spaced away from the firewall.
On a C-body if you put a PBooster on MBrake pedals you'll get a really soft pedal, that feels like it needs bled, you'll bleed about 2 gallons of fluid thru, you'll talk to 'experts' at a variety of automotive shops over a few months, get a variety of ideas, none of which work because nobody knows to ask if the pedal was changed when the car was converted from drum to disc. You'll live with it, because you have no choice -- and also because the car will outbrake every car in the family, including a new 91 Camaro. AMHIK.
Then Al gore invents the internet, and you eventually stumble across the answer.