If this is a K series engine, the idle speed is not adjustable. If the engine will not idle at 750-800RPM, there is a fault that must be corrected.
If it is an H20 with conventional Impco LPG system, there will be an idle screw on the throttle arm of the mixer. Ensure timing is at 10BTDC, intake and exhaust valves are adjusted properly, fuel mixture is set properly, then adjust to 750-800RPM. If the truck has a throttle dashpot on the opposite end of the throttle shaft, adjust the dashpot to 1200RPM with vacuum disconnected and ensure it releases completely with vacuum applied.