Jerry is indeen correct. Up until around 1958 to 62 most makers used the engine's # as the VIN for titling purposes. After that the formal VIN plate was used. In the event you are still having a hard time finding a VIN here are some spots to look at.
1- In the engine bay, on the firewall, in front of the driver, just below the lip of the hood. If the master cylinder is still there, look to the left (looking from the front to rear) of it. It will either be a formal plate type tag rivited to the firewall or a impact stamped series of letter and numbers. These may be hidden by corrosion or spray paint, depending on age and previous owner's.
2- Inside the cab, behind the driver, just above the floor. These one's were known to have been spray painted over during the building process.
3- Some have actually been put in on a body kick panel, drivers side, forward of the door, next to the starter/clutch pedal. These one's are usually found on International's and are, for the most part, usually rusted out and completely un-usable.
4- On the frame, between the door and the front wheel, either side of the frame's cab supporting crossmember, on the inside. This one is a toughie as it is almost certainly corroded to the point of oblivion due to road corrosion and frame rust. But due to the frame's material (this was in the days of real steel) and the impact tool's force when hitting the frame this one might have survived, provided you are willing to go after it. If corrosion is a problem let me know and I can give you some ideas on how to deal w/ it, safely.