There were many reasons for big gas burners in the mid 60's. Initial cost was probably the biggest reason. Repairs were much cheaper on gas burners than Diesels and there were many more mechanics qualified to repair gas burners than Diesel. The power band on a gas burner was 5 to 600 rpm while Diesels on those days were 300 rpm which required more speeds. The V12 had more power than the largest Detroit offered in GMC in those days and went as far if not farther than a Detroit between overhauls. The down side was that they got 2 miles a compared to 4 with a Detroit. But with 20¢ gas and 20¢ Diesel, carriers sacrificed fuel economy for initial cost and maintenance. Carriers who required more power paid much more money for something like a Pete or KW which offered Cummins up to about 380 HP. But the entire truck had to be engineered to handle this kind of power. Hauling bulk commodities between central Texas and the midwest, I didn't see the need for a Diesel until the late 60's. The V12 never saw much popularity because the 409 Chevrolet would almost put out as much HP at 4,000 RPM as the V12 at 2400 and could be bought at a fraction of the cost of a V12.