All the MCs back then were 4-bolt, whether the 66-older single-pot or the 67-up doubles, I believe they stayed with 4-bolts until the late 70s or so when they switched to aluminum (and the boosters were then updated to match)
C-bodies use different pedal bracketry between MB and PB, with a different pedal ratio. You either need the conversion setup (shown below) or to get a PB pedal from under the dashboard of another car. The pedal rod for PB will be native to the booster.
In the white firewall pic above, we can clearly see 6 bolts - I just looked at 2 boosters and a mtg plate in my basement, and the upper bolts would locate above the MC studs in the picture. The boosters I checked were the 66-69 dbl-diaphragm disc booster and a 71-newer large-dia single diaphragm booster (both with the same pattern). So I believe that the mtg plate would need changed, and I'd bet that the extra holes would already be in teh firewall. (the factory likely wasn't stamping firewalls specific for brake options, they were already doing that based on AC!)
The picture below is an MBrake car wearing a PBrake conversion setup. I believe it is a factory field-retrofit setup that I have heard of, but cannot confirm because I've never seen one specifically documented as such. It could be aftermarket, but the bracketry looks old, and the booster looks like parts-store reman. A setup like this is seen on some 70s A-bodies with PBrake, maybe other Mopars of this era.
If doing this conversion, try to get this area sealed up with the foam-type seal the factory used, as this will help keep fumes from entering the car. Even if you don't have an exhaust leak, there is surely oil/grime near the hot exhaust. (the seal doesn't have to be perfect, just don't toss it out thinking it's not useful)
Also -
If converting a column-shift car, the 71-up single-diaphragm booster will block the shifter linkage from hitting 1st gear. It might have too much boost for drums anyway. If using the band-clamp style in the photo below, be aware that if you ever convert to discs you'll need to convert it to a disc-brake booster. A drumbrake booster is fine for normal driving, but in a heavy stop it does **NOT** have enough boost for the higher pressures that discs require. Engineers wouldn't have spec'd a different booster between drum and disc without a valid reason that would be able override the bean-counters - we should trust the engineers in this regard.
Now that I've said all that, here's an anecdote:
I converted my 65 Fury from manual drums to PDB back in 1992 or so, way before you could ask the internet how to do it. I used a 71 disc booster, and had to do a little hogging on the pedal bracket to get the booster snout to fit. (but when you're young, and there's 6 years difference between parts, it doesn't raise a red flag like it might for me now. But I got it in there. (didn't know about the pedal ratio thing, nor did any of the brake shops or Mopar experts I talked with in the months after.)
The problem was the brakes were spongy, and I couldn't get them bled out. The car stopped very well (on par with my sister's then-new Camaro) but the pedal was so soft, it surely had to have air in there. Bled a couple of gallons thru, doing drip bleed over a span of hours, everything you could think of. Spoke to many people about my brake conversion, and got lots of ideas to check out, but never found the solution. Learned about the pedal difference about 10-15 years later, and it made so much sense why the pedal was so soft.
Ironically, I did my 2nd conversion to Manual disc brakes, and I love that setup even better. Yes, the pedal is stiff, but no worse than Manual Drums, and the longer travel makes them linear feeling. And I can stand on them with both feet at 40mph and make your eyeballs stretch just a little.
1967 Chrysler 300 Series | eBay
