The cabs are the same for 3000, 4000 and 6000 series trucks within any year. The 4000 and 6000 series trucks have different fenders, hood, inner fenders and related parts.
I've been told the grill bars are the same but the brackets that hold them in place are different, but I don't know the truth of that.
And of course there are running gear differences between every series.
Also be aware the '51 has unique doors. Turn-down handles with wing windows are a '51 only item.
