For 1978 truck models with the 250 L6 engine and light-duty emissions, the EGR valve connects to "ported vacuum" through the EGR TVS (Thermal Vacuum Switch) - color added for clarity. "Ported vacuum" means that the vacuum signal is zero at idle and increases proportionally with throttle opening while the engine is running. The choke vacuum break (aka, pull-off) should be connected to a "manifold vacuum" port.
If your 1ME carburetor has only one vacuum port, it is not correct for an engine with EGR.