Torque and horsepower are not independent of each other. You can't have HP w/out Torque.
Dynos measure torque, and use a calculation to figure HP.
Torque is what moves the car, regardless of how fast you want to go. If your torque peak is at, say...3500 rpm, and you do some mods to increase it, you will automatically get more HP with it.
If you have, for example, 350 lb/ft of torque at 3500 rpms, and you increase that to 400 lb/ft at the same 3500, you will also have more HP to boot.
You can't make more or one at the same rpm without getting more of the other.
The reason larger engines make more torque at lower rpm than smaller ones, (e.g., big block vs. small blocks) is because they also make more HP at that lower rpm, too.
Seems to me I remember seeing a HP/Torque chart a long time ago that pretty much gave you your HP conversion as related to torque at any rpm.
Meaning that, most any gas engine, regardless of size, if it makes 400lb/ft at 2500 rpms, it will automatically have xxx amount of HP.
The reason that large engines can do this is simple....cubic inches. They can move more air in and out at lower rpms....therefore, they have more torque/hp at lower rpms. Smaller engines simply can't do it. So they rely on having more HP at a higher rpm, and gearing the car differently to match that power curve.
I'm sort of rambling here, but my basic point is, I suppose, that torque and HP are not independently obtained. But the reason they are advertised so much, particularly in truck commercials, is because everyone things torque is a big thing these days...and to a point, they're right. But the manufacturer know this, and play to this ignorance about power.