I'm sure others will disagree, But my opinion is that any engine that doesn't turn freely with a spray of oil in the cylinders and not much force to rotate the crank, needs to be opened up.
Head off, pan off, evaluate the rust in the cylinders.
It is very easy to score and scratch the cylinder walls or brake pistons by forcing. A bit of patience, cleaning and care can save hundreds of $$.
Soaking with PT blaster and then tapping a piston down an inch with a wooden block and hammer (unless the lower cyl walls are just as rusty) lets you clean and hone the walls before moving the piston back up.
It can make the difference between - I got it running and I can evaluate the rest of the truck or Engine to the machine shop or replace as item 1.
