There isn't one inscribed in stone WMS length for any D60 rear axle. They were used under too many kinds of vehicles, for too long of a time. I'm sure given a WMS you could find one to fit your needs, or at least come close.
Plus a HD is only "Heavy Duty" in the sense that the housing has an extra rib. It doesn't have bigger bearings or different gears or anything cool. It's just a heavier boat anchor ...and when was the last time you saw someone break a D60 housing (yeah, it's happened...but not alot, and lots o hp).