Cat 777 questions on build

[Cooper]

I know these are expensive, but would only need one. I wonder if they would be able to haul 20#s of dirt in the bed. I just guessing but 15 pounds , does that sound correct for 1:14 payload? Without looking at conversion?
http://www.afv-model.com/4711shop/pr...oducts_id=3727
I've never attempted making an axle let alone the part that locks. I'm thinking to start with tires, then build around them. As that's one thing I know I'm not going to scratch build. I started messing with planetary gears I pulled from the "upgrade" kit for the 4200 excavator. Plenty of reduction.