That'd be a great gh motor to try 1st Greg. I think 1st gear in the wdc trans 16:1 & 2nd is 11:1 with a 5000 rpm motor. I found that 2nd was a bit too quick and stalling the motor on not much grade. This 20:1 gh motor is closer to 2nd gear, but maybe slow enough to give the motor enough torque to push the load.
But the weakest link in the wdc adt is the 1st pinion of the front axle, it has to push the entire model! So even with a torquey gh motor, that'll be the next thing to BLOW! Junk the entire powertrain in that model and put a motor at each wheel... as there is really no space to fashion in a transfer case... or better yet, just throw it in the garbage, spare yourself the aggravation!
Been down with the flu for a couple wks, I haven't done much with the 850. I nearly got the shipping damage repaired, a bit more sanding / filing to do.