I started out with a Tamiya Mammoth to build a Cat 789. The Mammoth was a great place to start as it has a terrific chassis and axle setup and heavy duty axle springs. I made a new box and changed the Engine cover to more suit the bigger model. I also made the cylinders from copper water pipe and brass and steel rod for the cylinder rods. The biggest problem was trying to find proper seals as the o rings didn't last long with the heavy lift.
Here's a link to my build and I'm going to try a couple of rcbrmin multi stage cylinders to get mine going!
Rob
http://www.rctruckandconstruction.co...?t=1350&page=8