The dual-diaphragm was used until 70, 71 started the large single diaphragm.

To expand on soemthign already mentioned:
Do *not* use a drumbrake booster for discs. It will work OK for normal use, but under heavy braking it will not provide enough boost for discs, so your braking will be reduced from what it should be.

Hasn't been mentioned yet, but C-bodies use different pedal setups between MB and PB, so if going to PB make sure to use a PB pedal.

I have manual discs on my 68 Fury and the braking is very good and has similar feel to the manual drums (meaning longer pedal travel and good modulation).