Since your car has drums, it probably has a 4"x 5 bolt pattern hub. Late model brakes (73 & > ) have a 4½"x5 bolt pattern. If you change over to late model brakes, your existing wheels will not fit the larger bolt pattern on the hub.
You can carry two spares or convert the front to the four piston caliper as in Oklacarcollecto's post link.
You can carry two spares or swap out the rear end for one that has a 4½x5 bolt pattern. If this course is pursued, none of the existing wheels will fit. Fortunately, the aftermarket and used market is full of choices.
You may elect to keep the drums. Obviously, they will stop the vehicle safely. They are not as heat resistant as disk brakes and will lose effectiveness with heavy usage. They do cool down and regain effectiveness, but that's not a good position to be in when a quick stop is needed NOW! Drum brakes are not as moisture resistant as disk brakes. A disk brake recovers much quicker than a drum brake when the friction surface gets wet. Finally, disk brakes continually self-adjust for wear. Drum brakes can also be self-adjusting, but it happens mainly in strong stops from about 10 mph and up in reverse.
I drove a 66 Dart V-8 in Austin with manual drum brakes from 1988-1992. Other than having a mechanic miss the L stamped on the studs on the drivers' side of the car, I had no real problem. I avoided deep water in the roadway like the plague. When I could not avoid it, I dragged the brakes as I went through the water. When I was through the water, I made sure it was safe to try to come to a full stop. The first attempt would be a little darty (pardon the pun) as some drums would be more soaked than others. Once I felt like I had all four wheels more or less working as before, I'd motor on.
The next 66 Dart I had (92-00) was equipped with manual disk brakes. It always came though the water test without any darting about.
