The 2003 8.1 has maximum torque @ 3200 and mine feels plenty comfortable at 3700 up hill. 4200 is max horse power. Fully loaded it is very hard to keep mine @ 3200 down hill on a 7% grade----she revs more than that and I would wear out the brakes if I fought it too much---or went to 1st/2nd gear.
The worst thing to do is lug the engine----you need some revs.
I do all my own stunts
03 Dolphin LX 6355, Workhorse W22, 8.1 vortec, 04 CR-V, Blue Ox, Brake Pro----Norm, Barb and
Doc(He's a PhD)