Strickly speaking it won't have a VIN number unless one has been applied sometime since about 1981. VIN is an internationally defined standard that was first used in the US the early 80's. I don't know when they started using it in the UK.
People question the accuracy of articles on Wikipedia, but I think the one on VIN is pretty good.
Wikipedia VIN ArticleCrenwelge is right, in the US most trucks used the engine number as a serial number as late as '54 or '55 and maybe later in some states.
I've seen serial number plates on the cowl under the hood on trucks of this era. I'd look there for a body serial number. That number will tell you more about the truck.
In the State of Washington I know the rules for changing the number on the title to match the body. I don't have any idea what the UK rules might be.
