When I convert these trucks I always replace the evaporator, condenser and drier. If the compressor is still good I remove it and drain all the oil, flush the lines then install the correct amount of PAG oil in the compressor. I also install two small 10" , 1000 CFM fans on the condenser. Getting good air flow across the condenser is key to making them cool and keeping the high pressure readings down with the amount of refrigerant I'm going to put in it. I vacuum it for a couple hours then charge it with 3 cans of r134a.