Bill, Usually the inserts that come with the kit are long enough for most uses. You can also stack inserts in the hole if you need more threads (drive the first one to the bottom and then thread another insert in on top of it.) But for your application, I'd say that 9 threads would be fine (installed at the top of the hole.)
I hear what Jon says about the knock-offs, but I have used them successfully. I do have genuine Helicoils in various sizes though, and prefer them if readily available. I've even used Helicoils on stripped weld nuts (the one on the stake pocket for the tailgate hinges.)
