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

I read everywhere that Horse Power doesn't matter, it's torque and gear ratio that matter.

I have a Freightliner DD15 455HP, 13spd, 3.08 rear ratio (I am not the driver, I'm the owner).

(Edited after I saw a comment by another user @W923 )
Now I am looking to buy a 400HP with cummins engine.

Will a 400hp, DD15 or Cummins of the same torque perform just the same on flat terrain as well as hill? What difference will I notice from the 455hp?

 

I read everywhere that Horse Power doesn't matter, it's torque and gear ratio that matter.

I have a Freightliner DD15 455HP, 13spd, 3.08 rear ratio (I am not the driver, I'm the owner).

Now I am looking to buy a 400HP with cummins engine.
Will a 400hp, DD15 or Cummins of the same torque perform just the same on flat terrain as well as hill? What difference will I notice from the 455hp?

 

Thank you. Yes, I understand what you mean. But since I have a choice to buy higher HP, I am trying to understand the concept before I do and regret it later. There is this walmart truck 400hp Cummins ISX15 with 2.47 ratio. I want to buy it but I'm afraid it may not perform going up hills in some areas like Pensylvannia or sometime W Virginia.
You think that concern is overblown? Thanks

 

Thanks.
Derating? I am not sure what you mean. Does a lower HP engine derate more than a higher hp? I am not quite sure why you brought up deration here. Please elaborate. Thank you.