R-134 won't cool as well in a Mustang as well as R-12 due to the size of the standard condenser. I converted my '89 last summer and it does a fair job, but not as good as it was.
If you convert, basically, you should replace the evaporator/drier, flush the oil from the system, change the oil in the compressor and install the proper oil for R-134. If you can swing the cost of R-12, use it.
Go to
http://www.ackits.com and read the message boards and information about the proper way to do the conversion.