Fill plug or drain plug?
If it's a fill plug, it's much less work to just get extras when you go in, and replace the thing every few years. Second easiest is to get an earlier cover (if the fill plug is in the same place - measure from the closest screw hole, since that pattern didn't change) and replace it outright. Third would be to clean the cover, weld on a bung, and change it.
Either way, if you go with a screw-in plug, use a brass plug so it won't seize on you.
The D35 got used quite a bit in domestic compact pickups, and you can even find it as a front axle on some Ford compact 4WD applications (Ranger, Explorer, 4WD Aerostar.) So, you've got options.
I'm not sure if the fill hole moved when they went to the "butt plug," that's why I said to measure.
If you're after a drain plug (I've often thought of that myself,) just drill and weld/braze in a bung (1/4" NPT should be fine) and put your plug in there. It won't drain totally, but you'll get all but a couple of ounces out - and you'll have the plug reasonably protected from rock damage. This can be done front or rear without too much trouble, I'm sure.
