Getting there...
Headmaster gorilla mode
Back (smokestacks, side panels, front wheels, bumper)
Headmaster in robot mode
He can sit inside the cab! Had to hollow out the upper legs and redesign the floor to allow a hinged steering wheel. Headmaster partner doesn't have a proper seat (yet!), he's kinda floating inside the cab (at least his butt is), he's only attached by the lower holes on the back of his leg. Very tight fit, it fills out the cab front to rear (which explains why the steering wheels had to be outside the cab).
Truck mode
Too bad I couldn't make the windshield wider, leg hinges get in the way. Still need to refine both modes, but the bulk of the work has been done!