I too have "been there, done that". Mine was either plug no. 5 or 6. I do know that Ford has had problems with the Mod motors on the passenger side/RH cylinder heads. I did all the plug changes in my car and I never overtorqued the plugs as I used a TQ wrench and always kept the TQ on the lower end of the range. IIRC the Ford TQ is 12-18 ft/lb. It's a good idea to remove the plugs when the heads are warm, but not hot. When reinstaling always start them by hand to be certain that they're not cross threaded. My father has been working on aluminum cylinder heads for over 40 yrs. and I follow his procedures strictly. That includes NO antiseize although you can talk to 10 different people and get 10 different replies on that subject...
Installing a helicoil without removing the heads is very iffy. It can be done but you can be pretty well certain that the plug electrode is not in the proper position depth-wise, in the combustion chamber. When I blew mine out I went to 3 different dealers for quotes and got 3 different stories. 1 refused to do the job w/out removing the motor so the cylinder head could come off. The others refused to even consider quoting w/out having the car in their shop. 1 said they might be able to do it w/out removing the head.
When the motor died later (from a blown head gasket) I pulled it apart to see how good of a job I did. The plug came out with the helicoil left behind (so far, so good) but the helicoil insert has a fraction too deep. Now the plug wasn't touching the piston crown, but it was about 1/32" further down than the non helicoiled plugs. I was very carefull installing it with a spare cylinder head on the bench to reference how deep to go. I also made 2 special tools to swage the insert and measure the depth. Now I recently bought another Tour and when I went to change the plugs, lo and behold, 1 of them came out WITH a helicoil. Can you say scrap cylinder head?
Gimme' the good old days with cast iron heads, RWD and lots of room in the engine bay...