A Dana 60 HD has extra ribs running around the top of the casting. A Dana 61 is hard to identify externally. It does not have a 60 or 61 cast anywhere. Not all Dana 60s have a "60" casting either. Try to get a mid-late '60s casting Dana 60. They are a sure bet. The good 4-pinion Power-lock sure-grips were fairly common back then too.