There's no difference in most vintage freewheel hubs (like Campy Record) made for 120-128 spacing, except for the thickness of axle spacers themselves and their arrangement -- just adjust the drive-side spacer to fit the freewheel you want with enough clearance on that side, pad the L side out if necessary to fit the dropouts or some standard spacing and build the wheel centered on that axle. You can fit a 126 mm wheel easily enough in 120 mm dropouts by spreading a bit when installing the wheel (helps to have 3 arms/hands).