Can a 400HP truck carry 35K 53ft van load on steep grade easily?

He is assuming you’re going to reprogram the engine of the truck you posted about.
I assumed you were thinking about a second truck.
Don’t know who’s right but it’s not important.
I honestly doubt you will see much difference your talking about 55hp and I wouldn’t think you are working it that hard but I don’t know what a steep grade is to you.
I can tell you that on a m11 cummins that I ask a lot more from. going from 330 to 370 with the same torque rating made very little difference. I doubt you could tell especially if the wind was different from one day to another.
It’s a dog it pulls a lowboy typical gross between 95-130k but I get pretty close to 6mpg on a calm day loaded one way and empty the other

 

Its not going to win any races with those specs but it'll go up the hill.

Why would any manufacturer build a class 8 truck that can't handle the standard 80k lbs?

 

Horsepower maintains the speed, such as when climbing. Higher the horsepower, the higher the speed up the hill.

Torque accelerates the truck up to that speed; does not maintain it.

Always buy higher horsepower.

 

Lol like the old 238 Detroit's and the old 180hp non turbo Mack 673s

 

Back in the day I drove an R model Mack with a 237hp and a 5 speed. Ran tanker, always 79k and change. Did the job, but was terrible on hills. A 400hp will get the job done.

 

I have a 318 ISC cummins, 6 spd Alison auto, it pulls 49.6K lb up a hill with a gradient of 8 to 16%, at 25 mph.

I also have a 460hp paccar, 12 spd auto pulling 93.6k lb, it does the same speed up the same hill.

I follow a competitor who has 510 paccar ponies pulling the same weight with his 12 spd auto. We do the same on the hills just I presume he uses more fuel.

all I got.

 

Dragonfly truck. Drag up the hill & fly down the other side.