Gary a r2 tranny from rc4wd.com is a monster with a 55 turn motor. I have a tonka Winnebago I converted to rc that one of my son's would ride on while it pulled my other son on a trailer. It topped out at a brisk jogging speed and would run for 2+ hours.
Does a halftrak steer with the tracks as well as the front tires?
https://www.google.com/search?q=kyos..._L4GYyaouRM%3A
You could help steer like this cutter brake system
Sorry I don't know how to copy the pic so look at the link