I don't think its necessary to go 4 wheel disks. The rear drums are decent for the amount of the braking force that they provide, especially for a street driven car.
On the other hand, I would definitely upgrade to front disks. Over 70% of the braking force to slow your car is provided by the front brakes. Plus, for the price of rebuilding the front drums, you can probably get most of the conversion parts, since you can do it with all oem parts.
I would swap in a set of '73 up disk spindles. This will allow you to use the more common rotors available. It would also allow you to go to the later F/J/M 11.75" rotors if you wanted to. You'd need a set of '73 up spindles, a set of caliper brackets (either the 10.8" or 11.75" version), calipers to go with them (same regardless of rotor size), bearings, and a new master cylinder and proportioning valve (disk/drum).