Hey LF6!

You might do well to conduct a thorough radiator/cooling system flush. It is a bother, doing it properly, but probably will give you good results.

Another idea is to hose off the radiators from the engine compartment outward (back through the grill) to dislodge dust, insects, and Jimmy Hoffa particulate. All that stuff can get compacted in there pretty bad, and with the truck's minimal maintenance, probably a good idea.

The radiator flush I recomment is where you drain the coolant, add Peak radiator flush and water, then run the engine for a while. Flush the Peak, fill with just water, run to operating temperature, then drain again (after engine cools). You're now ready for antifreeze.

The timing issue is a contributing factor, but a lot of sediment and crud-inski is the likely cause for most of the over heating, in my view. Note my array: 1990 4G64.