There should be a small stainless steel plate spot welded to the drivers side door pillar with a number stamped on it. Than number is the body/chassis serial number, which is what the vin buster will decode.
There shouldn't be a number on the frame, or at least not one put there by the factory. The number you have on the frame more than likely was put there after the engine was changed, as it does appear that you might have a '52 engine in a '51 truck (not uncommon). There also is a possibility that the doors could have been changed, which is also not unusual.
