wheel clyliders 69 dart

Ran into the same problem with my car Snake, and I am positive my bolts are original to the car. I ended up screwing the bolts all the way in by hand (clean the threads with a tap first) then used an open end wrench to snug them down.
What holds the wheel cylinder to the backing plate is the fit of the cylinder into the opening of the backing plate. If the bolts alone were used to retain the cylinder, they would sheer instantly as soon as the brakes were applied. Having said that, the bolts only job is to hold the cylinder snug to the backing plate. They don't have to be very tight.

George