I am starting to clean up parts now, so figured I would start with the differential. A lot of the undercarriage parts have a healthy coating of dirt on them as you can see in the picture of the differential below. I think the combination of dirt and oil on the parts created a nice protective coating as there does not appear to be much rust....just a lot of oily dirt.
Typically I would leave the aluminum bare on the differential as I think that looks very cool, however, we are planning on making this as close to the original as possible so the entire diff, except for the stud threads, are painted black. Pics below.