1966 Dart Rear Brake Wheel Cylinders???

8.75" rear-ends were not common on A-bodies, so not surprising. BTW, you didn't even tell us the year and model. Perhaps look at C-bodies on rock (Newport, New Yorker) since all those used 8.75" rears. But, you also want the proper piston diameter for your car, or could go a bit larger if you have an adjustable proportioning valve to fine-tune it, which is good regardless since nobody runs the factory tires anymore.