GOT IT!!
I really wanted to use whatever primitive technology I had readily at hand. I clamped the lock plate in my bench vice so the hole that hosts the tumbler was protected from over-heating then fired up my MAP gas torch and heated the hole area until dull red, held it there a couple of minutes and allowed it to cool slowly so the tap and surrounding metal were (hopefully) annealed. Worked - Then I slowly ran a 3/32" drill bit through the tap, then a 7/64 and 1/8". Then I used s broken drill bit shank sharpened to a point to knock out all remaining bits of metal debris. Then I drilled out the hole until all evidence of the old threads and irregular shape was gone. That took me up to a #3 drill so I then tapped the hole 1/4 - 28, countersunk both ends of the hole slightly, silver-soldered a bolt in place and cut it off about 1/32" long on each end. Peened both ends of the insert, filed off the excess, marked the location for the new hole and drilled it. Will buy a new tap tomorrow and say a little prayer while I generously lube the tap and myself before turning it in oh-so-carefully.
Thanks for all the good advice in case, God forbid, I should ever do it again...