I just checked a 1942 Chevrolet Truck owners manual, it breaks the trucks into four catagories:
Light Delivery
3/4 ton
Heavy Duty
Schoolbus
It lists the tank size for the 1/2 ton trucks at 16 gallons, and the 3/4 ton and heavy duty truck tanks at 18 gallons.. with 'school bus' tank capacity at 20 gallons.
The two gallon difference, is so slight i'd never noticed it, and needed no modifications to put a 2 ton tank in a 1/2 ton.
