There is no drain plug. You can cut a hole (large drill bit) and weld in a pipe thread bung that clears the differential and ring gear; then install a flush three eighth inch hex drive or eighth inch drive pipe plug in the bottom of the housing to drain it in the future. Personally I would do it the old fashioned way. Pull wheels and tire in the back then the brake drums. drop the drive shaft, before you unbolt then pull the axles out of the housing. You can then drop the center section out and inspect the condition of the gears.
If it were me I would clean the housing (a lot of kerosene with catch pans to contain it) parts cleaning bushes and then replace any bearings that show any sign of wear or pitting from water getting over the decades. Set of new gaskets and some paint and it will look like new.
Of course if you are going to change the rear end out for a stronger one (options are an 8.5 inch ring gear rear end out of a 1971 and up GM full size car, a 12 bolt out of a 1965-'70 Impala, or a Dana 60 out from under a Ford one ton truck that has been shortened to fit with new 33 spline axles held in place by big ford bearings). I wouldn't bother with the cleaning or new bearings I would just flip it over and use the fill plug as the drain plug.