My last truck was rebuilt, they replaced almost everything, including the engine. When they did, they replaced the odometer, but not the hour meter, which still showed high hours.
I asked jatonka one time, since he's a M35 guru, about the true mileage of these trucks, and he said most trucks get several engines over their lives. He said you can bet money that most of our trucks have 200,000 or 300,000 miles on their frames, but you'd never know it because they are rebuilt all the time with new parts, and the odometer/hour meter will never reveal the true life of the vehicle.