UnklRod asks, "Is there any sealant I can use to stop the leak?"
This is not good. First question is, "What caused this crack?"
Looks to be where the end of a cluster gear shaft resides or an idler gear group shaft. Cannot remember which for those old three speeds. Either way, this crack might be more extensive than appears; micro fracture hard to see or covered with paint.
If this trans has been rebuilt after the crack fact, probably not a problem but your case is weaker in that area. Most likely cause was high stress or blowing up inside. Really should replace that case part of your transmission.
These cast iron cases can be brazed but there is a need to drain the oil to do this and grind down to clean metal. Cannot have oil under a braze repair and more oil will weep out during heating. This is problematic.
Quickest fix is arc welding, but also requires clean metal and as little oil present as possible. Arc welding will burn away any weeping oil.
As you know, welding will mess up the paint. Brazing causes an issue with your tailshaft housing gasket; lot of heat is required for brazing. Arc welding is a better choice but more messy, requires some grinding and cleaning.
A "maybe" cure is fiberglass epoxy resin like from Home Depot. This also requires clean shiny metal and no oil present. I would grind this down to good clean metal, a bit scratched up for adhesion, clean with lacquer thinner, cut out a matching patch of fiberglass cloth, mix my resin, wipe the crack area with acetone, then epoxy this fiberglass patch over the crack. Very important to clean with acetone just seconds before repairing.
A two inch by two inch fiberglass patch, or thereabouts, would probably hold and stop this leak. Might leak a little right at the crack but under the patch. While the rest of the patch strongly adheres, chances are good no oil will escape from under this patch. Applying several coats of resin with sanding between would be a very good idea. Time consuming but worth the effort. You can feather the edges, never will notice a patch is there.
Prime and paint, ready for a car show.
Give the fiberglass / resin method a shot first, then consider brazing or arc welding. For all methods, clean shiny base metal is a must; no paint residue.
Be gentle on this trans, I know you will. You have a weak area there, a delicate area. Probably will be OK but don't push this.
Billyray
