Mine is a 52 but I've owned a 1951 in the past and as I recall it was the same. I have one of the Spectra tanks and it fit perfectly. The only thing I recall about the filler neck is that when I replaced the rubber grommet I had to be careful to orient that grommet so that the angle was just right...that being the filler neck had to point right at the two filler tubes on the tank when I was finished or it clearly wasn't going to fit right. I do recall having to move it around a time or two but hopefully you can see in the two images attached it fit fine once I got that done.
~ Jon 1952 1/2 ton with 1959 235 | T5 with 3.07 rear end
