1966 Dart Rear Brake Wheel Cylinders???

The 66 wheel cylinder you have was probably for a 7.25" rear-end, which may be a different backing plate than for 8.75" rears like yours. As I recall, the rear wheel cylinders on my 1965 Dart (10" drums & 7.25" rear) have an oblong bulge, similar to what the post photo shows. The bulge fits tight into the hole in the backing plate as I recall, which helps secure it. I would change both sides to the better bolts if I was you. Did you try browsing Google Images for various backing plates?