Freightliner with turned up c12

I'd say an 8LL if it has to creep under load. The C12 is (in my opinion) the best mid size Cat engine ever made, good on power and fuel, but it's not a torque monster. Probably perfect for the terrain you're talking about.

 

8LL with a C12 and 3.90 to 4.10 rears is excellent for a dump truck has been my experience. Light weight engine, and LL gears will really crawl off road, but can go 65 mph on the highway easily too. The only down side I had was old OTR drivers weren’t in the habit of using the LL gears and didn’t understand the concept of what we were trying to do.

 

My truck that I drive all the time (out of 3 I own) is a 2000 Freightliner FLD with a C12. Amazing engine! 500k miles and going strong! Not sure about trans but I love my 10 speed.

 

I would skip a 8ll and go for a 9ll if you want crawling speed,
if you will do alot of highway and hills jump to 18 speed, same low as 8ll but can split gears, which will help mileage and steep pulls.
Lowest gear ratios
8ll 14.56
9ll 26.01
18 speed 15.06