I am a licensed AC guy but it's been a while since I worked on cars but what I do remember is that rebuilding a compressor is hit and miss, parts or lack of good ones is not easily identifiable even to a trained eye. I installed new ones only, comebacks are a terrible loser for the shop and for the customer.
Coils and fans inside and out need to be good and a hard 1 hour or longer vacuum pull is important. Running the AC every 2 weeks is advisable because water droplets are inside the system even with a good vacuum pull and water mixes with R-12 to form an acid. It sits in one spot and starts to eat a hole in the metal, so running it stirs it up so it ends up in a different spot.
Refrigerant needs to be weighed in to specs but R134 requires superheat since there is no Ford Spec and develops higher pressures at higher ambient temperatures, I would opt for a R-12 substitute instead of R134A of which use scroll compressors which can handle the pressures and even liquid slugging which will damage a piston compressor.
Correction: R134 requires sub-cooling to measure in the correct amount of refrigerant but even then it's better to weigh it in if the specs are available. The problem is winter when AC is used with the defrost cycle, although I don't know if mustangs do this automatically. Newer cars activate the AC and the heater to dry the warm air of moisture. A undercharge or overcharge when the condenser is cold, for example below freezing, can damage the compressor. There should be a temperature sender at the condenser to cut it off until the radiator warms it up. Using R134 as a retro fit requires a AC engineer and a testing program to determine the proper amount of refrigerant. There are three components to R134 and they leak at different rates which is why it's not advisable to top off such a system. If it's low on refrigerant then evacuate it on a pump down and weigh in a fresh batch which is what I do on my suburban. It has a slow leak that I don't want to deal with so every two years I refresh the system with 3 pounds.
The worst thing is when the return line or low side to the compressor line ices up which indicates it's changing state from a vapor to a liquid since it's removing heat then it could enter the compressor as a liquid and destroy the compressor. This is why a undercharged system is just as bad or worse than an overcharged system.
Here is a good tutorial if anyone is interested.
http://proaviation.com/wp-content/uploads/2014/01/ConversionAir-brochure-R134a-pres.pdf
