Ball Mount Drop

The right one is the one that makes the trailer level when towing. This means that the height of the trailer and the height of your truck must be taken into account. You will most CERTAINLY need a weight distributing hitch (optimally one with integrated anti sway system like "Reece duel cam" or "Equal-i-zer") so your best bet is to buy a shank that is fully adjustable to work with your WD hitch. REAL important issues will be the fully loaded tongue weight of the trailer and the gvwr of the trailer. To get the fully loaded tongue weight use 15% of the trailers GVWR and size your WD system for that weight without being under that weight. With your truck it would be EXTREMELY important that you consider how these weights fit into both your trucks available payload (door post payload numbers minus weight of anything else you would have in the truck like people, cargo, pets etc). You can then also subtract that fully loaded tongue weight to see if it is within the trucks rating. Negative number means it's too heavy. It is equally important that you check the loaded tongue weight against the receivers maximum capacity (usually stamped on the receiver itself). Commonly half ton trucks have a maximum weight capacity (when using a WD system) of about a thousand pounds with some HD models going as high as 1,200 lb. They also have a maximum trailer weight (GVWR) of about 10,000 lb. with some HD models having higher weights.
Good luck / Skip