1st thought, You have not stated when the last time the truck has run? Has it had work completed since it was last run, like main bearing caps roped and shimmed? Can you turn the engine over by turning the fan by hand? Hopefuly yes?
2nd thought, Could there be corrosion internally in the cylinder walls?
If it hasnt been run in a long time, its not bad to pull all the plugs, squirt a few drops of oil in each and try turning it over with out the plugs. The engine should spin very well with no compression.
If you are still turning slowly and the answer to the 1st 2 thoughts are "no" you have starter issues and need a rebuild.
good luck, looks like you have a great truck!
