An 89 318 is not a magnum engine.

When you say overheating how did you decide that? Does it puke coolant? Or is the dash gauge what you are going by? Or do you have another temp measure device you are using?

Overheating at slow or no vehicle speeds are usually airflow related. Fan clutch, wrong (CCW) fan, gunk in the condenser or radiator, or both. No shroud. Idle is when the engine is least stressed so it is not making as much heat.

I'd double check the cam timing because someone has been in there. Replace all the freeze plus because it one or two are weeping they all will be soon. Put it together, get it running check timing (retarded can cause overheating), look for vacuum leaks (lean can cause overheating) and see what you have.