I'm in suburban Denver and use my 400 for pretty much the exact same stuff as you - hauling firewood through the mountains back to my house in the lower elevations.

I don't have the Aussie heads but have the TMI pistons & RV cam & headers & Holley Truck Avenger 670 cfm with the T18 transmission and 4WD.

I don't remember where my base timing is set at right now (I think around 14) but I'm presently running Shell's mid-grade fuel and it's pinging on low-RPM load with an empty bed.

Will most likely retard the timing a bit and play around to get it to stop, the high-octane gas is expensive.

IOW yes, pre-detonation will be something you'll have to deal with.

BTW in the Denver metro area, our fuel grades are generally:

85
87
91

Adjusted for altitude, of course.