Read the "SSAutochrome Headers Installed - Great" thread and I think you will find what you need. The SS seem to be a great value and they've fixed early production problems with the O2 sensor location.
I installed the SS Autochromes about a week ago, used oem gaskets and have no leaks, I also reset the ecu and havent really driven it enough to tell if there is a difference (2002 w/34k)-I maybe drive 20 miles a week. I did alot of research b4 buying the headers. I went to 4wheel parts and looked at the jba's b4 buying and the quality of the welds turned me away from them.
Hopes this helps.
No, I didnt see the need for it. especially for 100 clams extra. Some folks have installed them and feel like they lost power, also with the Y pipe you'll have to weld it in place, without the pipe it's straight bolt on. Just be ready to bust some knuckles and bleed a little when you install them. I am finally healing up.
I see that you have an 04 I'm not sure what the exhaust looks like so the Y pipe may be needed--maybe someone else can elaborate.