Brian,
In the bullnose trucks, there is a huge difference between Factory A/C and Non-A/C trucks. To convert to Factory A/C not only do you need the engine side box for the evap, which will also move the heater core into the cab, you will also need the entire dash and air plenum assembly. The Non-A/C trucks do not have the plenum or dash vents for the A/C. So you need all the parts on BOTH sides of the firewall.