Otherwise, healthy, intact 455, still in the car.
Started changing plugs only to discover the previous owner neglected to mention
he had stripped several spark plug holes.
Two questions:
Best way to repair? Heli-coils that won't back out? Other?
Is repair properly doable without disassembling the engine?
Hate to pull the heads just to repair stripped threads,
but will if repair in place is not advisable.
Also hate to drop shavings into cylinders.
Seems like I heard using grease on the tap would catch the shavings.

Seems like there was a thread on this subject some time back.
Did a search without success.

 

This is all you need:

http://www.timesert.com/html/sparkplug.html

I have used these countless times in aluminum heads with no problems or comebacks.
Yes, grease the tap to catch the shavings and you are good to go.
Good luck!

 

Use anti sieze compound and only remove plugs when cold for aluminum heads.

 

The Ford 4.6 V8's are known to blow out plugs, Timeserts are the recommended fix.