Changing The Heater Box

If I had to guess, and a visual inspection showed them to be the same, I'd guess the two heater boxes are the same. A quick look at either of them for a Ford part number would be the best bet, however. A quick call with a part number to the local Ford parts dept. would probably give you a more definitive answer. As for the A/C... that could get expensive. Most shops these days are reluctant to charge a system with R12, if they even have any at their disposal anymore. Since R12 hasn't been manufactured in around 10-12 years, the supply is low, and you need special CFC certifications from the EPA, just to get your hands on some. A more cost effective alternative would probably be to pick up an R134a conversion kit, and have a go with that. Talk to jrichker about it, he's very knowlegable on the subject, and has an excellent write up on his website at http://bellsouthpwp.net/l/r/lrichker/Mustang_tech/R134a_conversion/r134a_conversion.htm
Good luck!