Typically, the A/C system does not need to be evacuated prior to removing the A/C pressure switch. This is because there is a Schrader valve built into the switch mount. Unscrew the transducer and install the new one.

With those pressures you should not have to jump the low pressure switch. Start the truck put the fan on high, charge through the lowside. If your clutch isn't engaging, I suspect a faulty low pressure switch, the wiring in your system or faulty logic commands from the in dash control unit. I'm new ...