With the help of much advice from the wonderful folks on this forum, I managed to fix my stripped out spark plug threads - that had been previously tapped and fitted with a helicoil - by cutting new threads and inserting a time-sert.

The helicoil threading was of a smaller diameter than the threads cut by the the Time-sert tap. I was fortunate that there was just enough metal left to cut the threads for the 14mm x 1.25 Time-sert.

The Time-sert extended kit (Part 4412E) provides a tap and wrench that are supposed to reach down through the deep cylinder and into the head, but it's not long enough to allow easy use of the included wrench handle (it keeps bumping into various parts on top of the valve cover). However, the wrench attachment conveniently has a 1/2 hex head, so I just used a socket on it instead of the handle.

It was difficult to extract the seat cutting tool. It fits over the tap after you've cut the new threads, and cuts a seat easily, but it is difficult to find a way to pull it out of the deep cylinder. I managed to use a thin pick tool to push the seat cutter against the wall of the cylinder, and pull it up and out. This was an inelegant solution, but I could not figure an easier way.

A generous amount of grease on the tap caught all the tiny metal shavings. However, the grease proved difficult to clean out of the hole afterward. I used the aforementioned pick tool covered in a blue mechanics towel to pick up as much of the grease as I could. After many passes I got it much cleaner, but could not get all the grease off. I hope the remaining grease doesn't allow the insert to come loose.

A little oil on the insert installation tool ensured that the insert easily screwed into the head and did not come back up with the tool as I removed it.

Based on the recommendations on this board, I torqued the new plug to 22 ft-lbs. Afterward the car started right up, and has run beautifully since. Here's hoping it stays that way.

 

Congratulations, nothing beats success!