[Coley]

Ok I'll weigh in with a possibility....you might not be getting full needed voltage to the HEI/coil (equivalent) at lower/idle speeds, noting that you list an HEI unit as the distributor.
If you aren't....then the engine will run like it misses at lower speeds and can be hard to start.
Anything above 2500-2800 rpm starts to smooth things out as the alternator puts out more voltage to compensate for the low end, low speed voltage deficiency.

You can test this by running a 'hot' 12 volt wire direct from the battery to your hei or coil....then start up the truck and see how it runs.
This will essentially bypass any original wiring that might be deficient or failing to provide the voltage lower down.

All good
Coley