[AROWHED]

You can convert your 65-66 B body Pontiac to a factory 67-70 front disc brake system, but you must change out the spindles too which is a big hassle, plus a realignment may be required afterwards. But the drawback to that approach is it's about impossible to find new integral hub front disc brake rotors to fit the 67-70 Pontiac-only system. So sourcing a FLOATING disc brake rotor that could replace ONLY the drum, and leave the drum spindles and drum-removed hubs still in place would be ideal.
It's not easy to find disc brake rotor dimensions, but after a lot of searching, 2014-2024 Ram Promaster 1500 front disc rotors come very close.

Pros:
1. The Ram rotors are ALMOST a dimensional match to 67-70 Pontiac rotors, so they (can be made to) work, and are inexpensive through Rockauto.

2. There is no need to change out spindles, bearings, steering arms....so alignment is unaffected.

Cons:
1. You will need 67-70 B body disc caliper brackets, calipers, splash shields, and the special disc spindle anchor bolt that the caliper bracket requires. That bolt's thread does match and works with the drum spindle. And of course all the brake plumbing changes that any front disc conversion always requires: dual master, prop valve, etc.

2. The 65-66 drum hub will NOT fit inside the rotor hat until you lathe turn roughly .100" off the hub outer diameter. There is plenty of meat there so it's no concern.

3. The Ram rotors are 5 on 130mm [5.118"] lug pattern. That is so close to 5 on 5.000" that the rotor will slide on (since the Ram rotor lug bolt holes are .700" diameter) but the lug bolts won't be centered in the holes. So obviously it is best if you have the rotors professionally redrilled for 5.000". Another reason to do so is....

4. The B body drum hub has a 3.050" register, the Ram rotor has a 3.150" register hole. So the rotor will not be axially centered that way, but it could be centered by carefully matching the reduced drum hub outer diameter to the Ram rotor hat inner diameter, since it is fully machined there. Or ensure your newly drilled lug bolt pattern is axially centered and very close match to lug bolt diameter.

5. The .275" thick disc rotor may require slightly longer lug bolts. And your front tires will gain about .200" distance between them. Wide-Track!

Pictures show lathe-turned 66 drum hub on 66 drum spindle, misalignment of 5 on 5" lug bolts on 5 on 130mm Ram rotor, 69 caliper bracket on 66 drum spindle, and all of it assembled. Brake pad contact and caliper postioning is perfect.