[68Gold/white]

I believe that the attachment points (on the brake pedal arm)for the booster pushrod or the m/cyl rod (on manual brakes) is different. The manual brakes needing more leverage because there's more effort (as in from your leg!) for manual brakes, the booster obviously needs less effort, and might even be touchier if the push rod is mounted to the same place as manual brakes.

My 67 came w/ manual drum brakes, and my 68 came w/ power brakes, I plan to check the 67 soon to verify. I don't know if all the brake pedals had 2 holes drilled in them, for manual or power brakes...
If so, the manual brake push rod would be closer (as in higher) to the pivot point to get better leverage.

I plan to convert both my 67 and 68 to manual disc (5lug) when they get back on the road.

In my opinion, rear disc is not needed, it IS a better deal, but just having factory front disc brakes is 1000% better brakes, period!!!

If there's something better about drum brakes, I am clueless as to what it is. Drum brakes fade faster, need adjustment, even with automatic adjusters, wear fast, especially on the front, and can be a doozy to get stopped when wet. No worries about these issues with disc brakes...