Used truck specs

40k rears, not a hope in hell. Way too light IMO. Super 40's maybe but I'd personally go with 46k rears. Ratio I'm not sure what would be best but most stuff I see pulling that kind of weight with C-15's and ISX's have 4.10's or 4.33's.

 

40k rears will be way too light duty for that application.

A lot of chip trucks here in bc run 3.91s with 15 or 16 litre Detroit's and 11r22.5 rubber. There's a handful with 4.10s left which was very common when the bigger company's had 90km/h speed caps.

IMO, the 12.7 isn't gonna put out the mustard you need to get the job done. We ran 14l series 60 515s for a while and those struggled and were a pain in the arse (mind you that was back when EGR first came in)

Since youre looking for something older, you really can't go wrong with 6nz c15 550 cat. With 4.10s it'll climb a tree, 3.91s probably your best bet for well rounded versatiliy. Going off memory so you would definitely wanna fact check getting down to the wire but I believe you'd be cruising at 100km/h around 1450 rpm (with 3.91s)

Another good option to explore would be the N14.

 

I think the biggest issue you'll have is finding one thats not beat to death or just worn out. 500 plus hp /18 spd/ super 40"s with 3.73 to 3.90. If your going in the bush 4 way lock ups and 46's would be preferable. Look for something with a heavy single frame because of the corrosion issues with double frames. Cat,cummins,detroit. I would mack and volvo even with the detroit. Good luck finding one at a reasonable price.

 

A 12.7 can put out the power needed, but not stock.

 

Any one I knew pulling those weights with a 12.7 usually had 4.33 ratio.
The 40,000 lbs rears can be used if you are staying on the pavement or hard surfaced roads. If you're going out into the bush they won't last.
You'll want an 18 spd as it has lower bottom gears to get you going in softer terrain.

 

Being from Quebec he has many great options to tune that 12.7 to do the job .. many chip and log trucks have the 12.7 the main issue is the weaker jake brakes compared to a cat or cummins