The #3 often gets stripped because the tach cable drive is right there. It is made for easy removal so the plug can go in/out, so while you're working in there, take a look at how you do that, so you can save yourself this grief later.
Something important to know in the 350/400 head when installing Helicoils: do not drill all the way thru the hole to tap to the last thread. Instead, stop about 1-2 threads short of the end and install the new threads above there. This will prevent something I have seen in the 350/400/500/550 engines before where installing the whole threaded length can also let a cock-eyed sparkplug force those threads down into the cylinder head, one day in the future, and then the threads and piston collide. Very expensive to fix!
And, it happens in the #3 cylinder, because the wrenched was too lazy (or didn't know) to remove the tach cable before installing the sparkplug.
BTW: this happens on the 500/550 more often than on the 350F/400F, in my experience - or, maybe because there are more of them out there?
