Bill
Looking at your photo the thread damage does not look to severe also looking at the unafected thread area I would say, espeacially after running a tap through, that you would not have a problem if the plunger was screwed in and locked up tight, as the area of good thread looks to be sufficient.
If it was my gearbox I would refit it and ride it after all if the problem comes up again then that would be the time to change the housing.
I would not bother making a bracket to support it as if it holds when the locknut is done up then it is unlikley to shift
I am not qualified to give an experts opinion but this is my own
Regards
Richard