Despite what the mopar book says, 62-65, 66-67, and 68-70 b body rears are all different widths. This will directly impact where the wheel assembly ends up which will then impact rim offset selection.
Very true, but the '70 he says he's using is the widest of all of them. Unless it's not actually a '70 B body rear end. A '68-'70 8 3/4 B rear should be 60 1/8" drum to drum (54 15/16" flange to flange), a 65-67 8 3/4 B rear should be 59 1/2" drum to drum (54 1/4" flange to flange).
'62-63 and '64 were also different widths, but I usually don't pay much attention to those because they have tapered axles, the kind that has the drums retained by a nut on a taper with a keyway.
So it very well could be that B body rear wasn't actually '70, but a 65-67, which would need less backspace.