Have you checked the timing with the spout pulled? It should be 10 BTDC. It may be past time to change your plug wires. Mine ran so much smoother after I changed the wires.

Bruce

 

My 5.0 idles 650, pretty sure the 5.8 is the same at 650-850

 

Thanks for the input on plug wires - I was thinking about that. I did sandpaper the contacts inside the distributor cap to clean them up. Pretty oxidized so probably the orig cap (and wires). Yes, I’m used to the idle being around 650. I haven’t purchased a service manual for the truck yet so don’t know what it’s supposed to be.

 

I have a new theory, the engine should idle at 650 when warm, it tends to idle around 800-900 when still cold.

I'm beginning to suspect your ECT sensor isn't functioning properly. This may explain the rough idle, as it may actually be running rich, thinking the engine is colder than it actually is.