Air conditioning unit

You didn't say what car you are working on.
On most of our cars you can look at the firewall and tell if it had factory a/c. Non a/c cars had two holes for the heater hoses. A/c cars had two additional holes to accommodate the a/c lines.
You might contact member CharlieS - he has /6 brackets and a wealth of knowledge for installing a/c into /6 cars.
C