13 Speed cut it?

I think the 13 sp will do fine.
Most of the time you are going to be in the top half of the gearbox even on the grades.
As you know the splits are the same in the top half so no difference except that the Pete has shorter rears so this is better on grades.

 

The 13 will be just fine as you have the same ratios in Hi range.

 

Unless your entire run or at least half of it is spent in serious mountain grades with that kind of weight, a 18 speed is utterly useless. There is no reason to ever split the basement unless you are starting on a grade or in some very serious hills with some serious weight.

Personally with that kind of weight, I would not entertain a truck with less than 550HP, I would be looking for a 600 or 625 CAT. Upgrade the cooling system and enjoy driving a real truck.

 

You could probably uprate that 475 anyways.

 

Kind of sounds like the run for Day. Vale and Xstrata ore haul??

If so the white 05 flat top pete with blue roof is a 13 speed. He hauls it.

 

Crank it up to a 550.

 

He's only going to be able to uprate a 475 up to a 500 through Cummins.
Anything above that will require a change in hard parts, not just the ECM.

Aftermarket (PDI, J-Ball, etc.) might be able to get more than 500 out of it without hard part changes, but I'm not sure about them one way or the other.