I just went and measured the bed, and it's bigger in every dimension. I'll bet it's a 53 long bed that has been cut down. They did a decent job...it's not real obvious where it's been cut. I would bet that they cut it off the tail end of the bed.
You know, all of this makes me want to double check the wheel base to make sure this isn't even more of a Frankenstein's monster. Does anyone know the wheel base of a '51 F1 off hand?