In 1951, the smaller trucks came standard with a 216 engine; and, a 235 engine was not available as a factory option, except for 4000 series trucks.
Your 6400 series truck would have come from the factory with a 235 engine.
Here are the details. You can usually ID the year of a low-pressure 216/235 by the Engine Serial Number that is pressed into the block to the rear of the distributor.
Obtain and post that code and you will get help/confirmation on the size/year engine.
