Does the Title list it as a 3200 and you said the title vin matches the vin on the plate? If this is true I would think that is your official documentation. It is recorded on the title and the state has signed off on it. This could have happened before the state started issuing titles. Unless California was unique, in 1957 states had not gone to the title system yet. I would rely on the state documentation and the fact it matches the truck and not the 60 year old manufacture guide even though it is correct, a lot happens to these vehicles over the years.

Dave from Northern Kentucky
My 54 3100