Convert ST tires to LT tires

I'd do that by going to a couple different tire stores and discussing the matter with the tire guys. They have reference charts that give data such as rolling diameter, weight limits, etc. that you will be hard-pressed to find otherwise. Those details are more important than any mere size designation. What's more, you need to understand that tire sizes are not quite as standard as say wrench sizes. Sometimes changing tire brand requires a different size to get a better match.