Yup, that's why the horsepower rating exists. A turbo 4-banger making 400 hp is capable of moving a load just as well as a 400 hp Diesel in a Superduty, if you geared the thing low enough to accelerate up to speed. However, it's not the right application, as going cross-country at 8000 rpm is probably not a recipe for reliability. Both engines are just as powerful, they just have different operating characteristics.The torque line isn't what matters, it's the HP curve and it goes up all the way to redline. Despite your obvious belief, horsepower provides the acceleration and speed holding, not torque.
A conversation about what I can and can't afford isn't relevant, and would have an unfavorable outcome for you. I will say my M5 and my wife's Range Rover have similar RPM in their top gear and also run stock gearsets. And even though they both are far more powerful than my JL, they downshift to accelerate at highway speeds.
Torque may be relatively flat on the 3.6 from 2000-6500 rpm, but try timing the rate of acceleration. See how long it takes to run from 2000 to 3000 rpm, 3000-4000, and 5500-6500 rpm. You will find that the horsepower the engine is making at high rpm cuts the rate of acceleration significantly. 2nd gear is probably 1-1.5 seconds from 2000 to 3000 rpm but 5500-6500 is less than half a second.
That's the beauty of selectable gears, the gear you choose for a give mph determines the horsepower at your disposal