a putty knife to cut the gunk would be a good start, then some solvent and a rag should clean up the rest. Mineral spirits is easily obtained at any hardware stor. I have some really cheep 1-2" brushes for this kind of work. If you have stands get her up in the air with a pan (partially filled with solvent) under the differential and scrub it clean. I don't know that you need a gasket to fix the leaky diff, I have had good luck with a simple bead of rtv and have been good to go. Very nice car and welcome to the world of old cars...oh and to the site
