You don't want to use a generic chi-com o-ring if you can avoid it.

The dealer may have an assortment of the proper o-rings, or a good local parts store will have an A/C o-ring assortment in a bin that you can use to individually match them up. Most General Motors, Jeep, and Ford o-rings have been metric for quite a while. The Green or blue o-rings are suitable for R134A.

Remove the o-rings and bring in your old ones to match them up. I would suggest either tagging them to their location, or putting them in individual zip lock bags with the location written on the bag so you can keep them in the proper location. If the connections are threaded on your truck, it helps to soak the connections with your favorite penetrating oil the night before you take the system apart.

I would recommend using a new receiver-drier/accumulator, if at all possible, especially when converting the system over to R134A from R12. Also, blow out the condenser and evaporator coil to remove the old mineral oil. If you see any metallic particles in the old oil, cleaning the system is especially needed, as the metallic particles indicate that the old compressor is coming apart internally. There is a flushing solvent available to help remove the old oil. You flush the fluid through with a siphon gun and air pressure.

Some compressor designs allow you to siphon off any accumulated oil from the crankcase, too. Too much oil takes up space and lowers the efficiency of the system, so if you can remove as much oil as possible before you add the new oil, the system will work better. Some compressors can be removed from the engine, then turned upside down and drained through a plug in the side, if you can't siphon it out.

Be sure to use R134A refrigerant compatable oil, usually I use 8 oz Ester oil when I convert a vehicle. Sometimes the OEM has a TSB covering the conversion, like Chrysler did for the older models, that lists the recommended conversion oil and the amount needed.

Some shops will use pressurized nitrogen or CO2 to proof-test the system. They will charge it up then watch the pressure for signs of leakage/loss over an hour or two. The nitrogen will also dilute any air/moisture in the system, before they go ahead and evacuate it for recharge with the R134A. If your shop used nitrogen to proof test the system, they know what they are doing! They should have and probably did evacuate the system at at least 28" of vacuum for at least an hour before the system is charged. (This will pull any residual moisture out of any remaining oil). If nitrogen or CO2 was left in the system with no refrigerant, you would have no cooling effect since neither gas will have enough pressure from the AC compressor to convert back to a liquid state in the condenser. The rapid change from liquid to gas through the H-valve or orifice tube restriction is what causes the cooling effect in the evaporator coil.

Hope this helps!