I've got one book that says the Idle is not adjustable - and one that says it is adjustable.

There is a screw on top of the throttle body - that doesn't seem to have any affect on the idle speed.

Any Clues. I've just rebuilt this puppy and I am trying to adjust the timing but I need to do it at 800 RPM It is idling at about 650 RPM.

Other than that it is running like a swiss watch.

 

The screw should be in the side of the TB pointing towards the front.
Adjusting the screw should change the idle alot.
Are you checking the idle with the truck in neutral?