d46 rears with 3.36 ratio

Why?

 

Cheapest way to increase engine rpm without the expense of changing differential gears.

 

Wouldn't it be cheaper yet to use the transmission?

 

I concur, leave it as is and see how it works out.
You can always drop down 1 or 2 gears and still be at reasonable highway speed.

A friend of mine pulls super B's with 3:36 rears, but he has a jacked up C-15 that makes about 700 hp and big torque, with the heavy 18 spd transmission I'm assuming, don't think it's the 1850 torque rating, but I could be wrong.
He runs MB and SK so no telling how it would be in the mountains.

The 46k rears are only about 50 lbs or so heavier than the 40k rears.
Only worry I might have is how the clutch will hold out if you're on soft ground a lot or starting up a grade from a stop.

 

You got lots of gears, keep your revs at 1700 or higher, top end of power band, when climbing and start your descent slowly, with your 4 ways on, in a lower gear with the jakes on and see how it feels, feel out the situation pick up a gear or two if feel safe.

If you stop, do it on flat ground or a forward down slope, get a little fuel economy from gravity.

Back in the 80's, I once did 2 rounders in 7 days from Wpg to Vancouver in a truck with 400 hp and no jakes, through the canyon to avoid the Coq.
The highways are a lot better and the trucks these days are a lot safer with a heap more power.

You'll do fine, get out there and check out a little scenery, do some serious trucking, enjoy the view and relax.

 

Low gear on the low side he'll be fine. Even with 3.36 the start-ability will be fine