Wow, that is the engine in that truck? Man, somebody was taking care of that.
I used something called Fluid Film in my cylinders. It comes in a spray can and I find it the best rust breaker that I have used. I jacked up one rear wheel and used it to turn the engine over. I started with just a few degrees of movement and after a day or so, I made it through a whole revolution. Next I used the fan and belt and the last thing I did was turn it over with the plugs out, using the starter. You might want to pull the distributor and turn the oil pump over. Also if it stays stuck, I would pull the valve cover and take the pushrods out. If it is some stuck valves holding things up, you will soon know.
