Alwyn what kind of motors will you be using, I ask because I am fitting two N20 Miniature High Torque Electric Gearbox Motors from ---
www.banggood.com
for £2.95 Freepost each and fitting them into a twin motor mount from "Wayne",
One motor driving the front axle and the other driving the rear axle, these geared motors are available with various "RPM", I have selected 160rpm,
Choosing geared motors avoids the need for the transfer box,
Various lengths of metal propeller shaft in 1/16 are available should you need a longer or shorter version of the H/L one.
Shaun.