6-ply or 10-ply?

Tires haven't been rated in "ply" since before dirt was invented. Tires are now rated in a load range designated by a letter. On the door jam of your vehicle you will find a label that will tell you the exact tire size, load rating, and tire pressure. That is what you should buy.

If the specified tires are not strong enough to carry the load you are putting on your truck, you need another truck designed to carry that load.