> If you can get a welder, an easy alternative maybe to
> simply weld a nut onto a lug bolt.

You don't even need a welder for that. Some flux, a brazing rod, and a
propane torch is all you need to attach the nut to the bolt.

And you don't even need that. Put two nuts on your lug bolt and tighten
them to each other. Now stick your bolt in your trans plug and turn on
the inner nut.