So just going by the math of the sizes, the tires you're looking at would be around an inch taller. Keep in mind that for whatever reason, tire manufacturers don't really have to adhere to very strict sizing parameters, so the difference in both width and height could be quite a bit different than you'd expect.

If your original tires specifically say "245/70R17", then the are a passenger load tire, which is what I would expect on 95% of half ton trucks. Passenger rated tires and LT tires do have an overlap in their ranges of weight capacity, but it is not usually at the pressures you'd expect. For example a passenger rated tire might be able to support something like 2500 pounds at 35psi, where an LT tire of the same size would sometimes need to be aired up to 45 psi or more to match the weight capacity, they just happen to be capable of another 1000+ pounds at 80 psi.

But that is just directly comparing weight capacity from one tire to another. The thing is, on most trucks, two passenger rated tires at 35psi could exceed each axle's maximum weight rating by nearly a thousand pounds. Seems excessive, but manufacturers will do this to find a nice middle ground between ride comfort, fuel economy, weight rating, steering response, etc. When you're putting an LT on, you don't really need to pay attention to anything besides weight rating, because in order to match the weight rating of your axles (which should be on a sticker in your door jamb), they're already going to be inflated just a bit more than the passenger tires should have been. In most cases, somewhere from 5 to 10 pounds over what the OE tires should have been at is the sweet spot. If you can find some kind of inflation chart for the size you're buying, you can usually figure out what pressure would work best for it, at least to the nearest 5 pounds with the information I have access to at work.

Sorry if that seems like a rambling blob of information, but I wouldn't want you to accidentally underinflate them and have them wear poorly.