Drum to disc swap is super easy - use an Explorer axle; use F-150 lowering brackets to change from spring under to spring over.
On a more serious note - swapping Mustang rear discs isn't that difficult but here are the cavaets:
Mustang discs are better than not functional drum brakes (and about 90% of drums I see haven't been maintained and are effectively non-functional).
Mustang discs are slight better than 9" drum brake when both are working good
Mustang discs are WAY worse than 10" drums when both are working as designed. Exception: the ebrake application of Mustang calipers is WAY better.
Explorer disc don't mount on Ranger axles but are about the same improvement as 10" drums are over 9" ones.
If you install rear discs you MUST change the residual pressure valve - drum brakes require 10psi, discs only 2. If you don't change them, you will go through rear pads like you owned shares in the company.
Lastly, the Mustang rear brake's ratchet mechanism for the ebrake, requires they work when by applied by a 90lb weakling using arm strength. When applied by 300lb lineman (aka my son in his Mustang) or foot pedal (aka my daughter in her Ranger); they won't release properly. So, for 1st few miles, you will be wearing off pad.
But they do add bling over drums.