LOOK at each spark plug you pulled out of the heads, if any of them have rust you have big problems. If you soak the cylinders with atf or wd-40 let them set 3 days. Then try to turn it . If you still can't turn it , drain the oil out of the pan , remove the pan and loosen one main cap at a time. Each time you do this , retighten the main cap to spec's if the engine does'nt turn free. If you've done all the mains and it still will not turn then do the rods the same way. If you find that you loosen a rod cap and it turns, STOP. Now check that cap and all others to make sure they were installed the correct way on the corresponding rod !! if all is correct and it does not turn buy hand the the rings are rusted to the cylinders...year it down.