Changing from R12 freon to 134A

The proper way to do this is to recover any R-12 left in the system. Disconnect ALL the lines and flush the parts with R134a to remove any excess oil. Then replace the drier, and accumulator. Also you may need different pressure switches depending on which compressor you have.

Personally I would replace the compressor with one that is rated for 134A (many available that work with both and are bolt ons)

Then you install the conversion fittings and charge the system using pressure gauges and temperature. You should end up with a charge of approximately 75-80 percent of the amount of R-12

If the system has leaked down the 134 will leak down faster. So replace/repair the leaking part.

DON'T use one of those kits you find in many parts stores without following the above. The ones that claim you just swap the fittings and add the 134A and you're done. They tend to cause more problems and compressor failure.

If done correctly the system will work and keep you cool. Do it the shortcut way and at best it may work halfway, at worst you can watch the compressor die quick.

I would use the 134A. Mainly because of the ability of others to work on it if you have a problem. Freeze 12 is a blend of 80% R-134A and 20% HCFC-142 as a blend it is not able to be recovered by current EPA standards. Also the 134A is cheaper than the Freeze 12 and will work as well or better. One thing you may want to look into since you need to replace a few parts anyway you may want to change the condensor to a larger 134A friendly unit. One from a later S series with 134A should bolt in without much work.
Parts wise the drier, replacement O rings and the condenser (if you wanted to replace it) plus the refrigerant and proper oil. If you're handy with a wrench you could do a lot of the work yourself.

Well if you have parts access you could grab a complete system off a 94 up P/U or 95 up S- Blazer. That would get you the correct hoses, condenser and brackets. I would probably grab the A/C compressor brackets and hardware to swap as well. Get it all bolted on and you would have a 134A system, if you grab it off a vehicle that is still holding a charge and cap the fittings ASAP it would make less work for you. You still will want new Orifice tube, O-rings, drier, because you really should replace them if you open the system and DEFINITELY if your switching refrigerants and not replacing the evaporator. The rest you could get from the donor vehicle. I'm not 100% sure but I think the evaporator core would even bolt in as well, It's a bit of work but if it's a vehicle you wanted to save it would be worth it to have the matched hardware instead of mix/match stuff.

. Just ask questions and we will point you to the knowledge.