You will need the manual brake firewall plate (for a '79-80 Aspen), along with the master cylinder pushrod, and then use a master for a manual brake '79-'80 Aspen or Volare . You'll also need to rejigger the master to combination valve lines because the master will be in a different location. If the firewall on a Mirada is different, the plate may need to be modified. I didn't actually change from power to manual, the car was built that way so hope I'm not wrong about what I wrote on the conversion
Napa has a reman unit shown (without reservoir) for $68, but also has a NEW one with reservoir for $38, go figure. The new one APPEARS to be for manual brakes but it doesn't specify.
For reference. The car will stop fine with manual brakes but fast stops require pretty good leg grunt
How the setup looks on my '79 Aspen: