Cool Dave’s 1973 Dodge D100 Club Cab

Just skimming through. It sure looks like an EGR valve on the intake. Off the top of my head EGR should be closed off when the engine is cold, at idle hot and cold, as well as at wide open throttle. Typcially they use a ported vac source and some sort of thermal switch usually in a coolant. vac diagram should be (was ?) in the engine bay. Some used a venturi vacuum signal. example diagram of venturi signal system

Some info on the EGR systems in the Master Techs series in '74. Can also skim back through '73 and '72 to see what is there.
https://mymopar.com/browse-mtsc-by-model-year-1974/
Knowing the engine has smog stuff on it, would want to see the carb and check the FSM or Sun cards to see what else it had. Generally trucks has less junk added to them compared to cars of the same year. Shop manual (cars) shows smogged 318 ought to be around TDC to 2.5* at 750 rpm. But that only works at higher rpm with the matching distributor curve. An earlier 318 with its distributor will be more like 5* at 650 rpm. Measure the rpms vs timing and we'll graph it.