I just tore into some longblocks that have sat for at least 15 years untouched in my MI polebarn. The cylinder walls didn't appear to get any worse than they were when put into storage. I just backed off the bolts that hold down the rocker rails when put into storage. How damp your barn gets may be another story, mine seems to get a lot of sunshine when we actually have it here.

To store a bare block.......I'd remove all the pistons and grease the cylinder walls up with wheelbearing grease as mentioned above. You can go indefinitely at that point. I'd leave the cam & lifters in the keep the lifter bores occupied with oiled up lifter bodies in there. The "fogger" cans of WD-40 sprayed over that is just cheap insurance.

What hurt my situation was the invasion of mice living in one longblock. Not good. The urine caused more rust than any humidity ever could.