Charles
Welcome to the world of GT40s
I would think you are unlikely to get a truck gearbox to work in the GT40.
In a Gt40 you need to have a transaxle where the gearbox anf differential are all built into a single case
Most trucks use a gearbox attached to the engine and a differential at the back axle - not combines so will not worl
that said if you have found a truck with a transaxe I'm sure a lot on this site will be interested
Regards
Ian