I recently retrofitted my '79 Bronco with a 400 Cleveland with modern A/C. It was not very difficult or very expensive. If you have a SBF, your brackets to mount a Sanden/Seltec compressor will be different, but for an engine originally supplied with a York/Tecumseh compressor you can find adapters that bolt on to the existing bracket. With a Sanden ($200) or Seltec HD ($300) compressor in place (your selection will depend on what you want; Seltec HD is heavier duty and less vibration, Sanden slightly cheaper, but more vibration), you'll then need to purchase a parallel flow high efficiency condenser ($60-80), along with a new binary switch ($25), and receiver/dryer ($40). Assuming your evaporator is in good condition, have it flushed properly and replace the expansion valve with a new unit designed to work specifically with R-134a. At this stage all you need is new custom made lines designed for R-134a and you're ready to go (assuming your wiring and vacuum lines are good).
The end result in my Bronco was incredibly cold A/C and I'm about to do the same on my '71.
