IWL BCNU,
On my 40 there was a way to adjust the contact plate on the starter. My truck has been running on 12 volts for over 30 years and I've had to do this several times to keep it working. The switch only slides a fraction of an inch but that should be enough to work. You also might try replacing the switch, as they do wearout from the arching when starting the truck.

Good luck.