How much work is involved to change out the master cylinder and power brake booster on a 1972 Mustang?
I am not familiar with the setup of your mustang, but I am going to assume it is similar to other booster/master setups I've seen. Here are the approximate steps you would follow.
Disconnect pedal from master/booster.
Disconnect vacuum line from booster.
Disconnect brake lines from master.
Disconnect booster from firewall.
Install is the reverse of removal.
Do your best to not spill the fluid, its corrosive and causes rust. You may want to remove as much fluid as possible before removing the old master.
Before installing the new master, bench bleed it. Run a line from the output of the master back into the reservoir and pump it until you don't see bubbles anymore. Keep fluid in it and make sure it doesn't run dry or you will have to bench bleed it again. You will need to bleed the system when you are done. This may take a while as you need to bleed for the entire length of the system.
I am not sure if it would be better to remove/install the booster and master as one or separate, or if it really matters.