Re: Couple questions for ya
I wouldn't worry about changing the headers unless you are changing the mid-pipe at the same time. The stock 5.0 headers actually aren't bad at all. They are actual headers instead of exhaust manifolds. You can gain a little bit of power by swapping them out, but it probably isn't going to be worth it on a mostly stock car. If you are going to be chaning the mid-pipe too though, you might as well do it all at once.
As for the timing, it's just something you have to play around with it. With the stock heads, I'd say that if I was going to do anything it would be to bump it up to 14 and run 93 (or 91 if that's what they have in your area). Of course, you should always listen very closely for any pinging. If you hear any, drop it back down a degree or two.