The higher the number for the load index, (not the load range), the higher the weight capacity. You are wise to go to LT tires from passenger, P tires, as the passenger tires are not as durable for a truck, especially when towing or hauling heavy loads. Those low profile tires can be a problem with heavier loading, and on our truck when we purchased it, the dealer had installed aftermarket wheels with the low profile tires, had to disclose it to us, and I refused to make the purchase until the original tires/wheels were put back on it. The following link has a chart showing all the load index numbers and what the weights are.
http://www.discounttire.com/dtcs/infoLoadIndex.dos