Scania 420 vs 360

That question is for @98989 .
But if speeds are that low then gearing down will help the underpowered engine.
A ex EU or UK truck has it sweetspot around 90k per hour.

 

How much floorspace do you need?

 

From what I gather in the conversation, frame and component strength aside, a 360 hp engine geared properly should have no problem hauling 56 tons at 70 km/h.

Back in 1978 I was pulling an A train at 135,000 lbs with a 350 hp Cummins over hilly terrain, (paved), and it did a decent job, but it was geared to do the job.
Road speed ar 2000 rpm was 58 or 60 mph, 15 spd direct transmission with 4:11 rears on 24.5 tires.

Most engines will give you decent fuel mileage if you gear them accordingly and don't drive the crap out of them.
A higher hp engine doesn't automatically use more fuel than a lesser engine, it's how you use the power that determines the fuel economy.

I have achieved nearly identical fuel economies with a 1995 Kenworth W900L, 3406E Cat engine @ 550 HP, as I did with a 2001 Volvo 780 with a D13 @ 460 HP.

Spare the horses, save some fuel.

 

How many pallets will you load?
How big are the pallets?cm x cm
If it was me i would stretch the truck but leave it as 6x2.
Get a trailer with 2 axles at the front ,2 at the back.
Lift the extra steer axle when empty on the truck.