From what I read recently the newer A/C systems are 30-35% more efficient than they used to be. Would it be possible to retrofit a newer system from a 2011-2012 Mustang to a 96-04 car? I think you could get a couple extra mpg and who knows how much hp/trq.
I think the main point of gain would be from the compressor being more efficient. Like 5L5 said, you can do it but you would need to be able to custom fab the bracketry. The only wiring would be the power to the clutch for the compressor to engage. You would likely need to take you AC hoses and have the ends redone to fit the new pump.
Also, both your AC pump and the new AC pump have a clutch on them so they only use "extra" power when you turn the AC on. Seems like a lot of work to me to only pickup power when the AC is on. I think money and time are better spent on other mods that giver better mpg or power all the time.