I picked up a used MAC prochamber off a local guy for free. Its not prefect but in good enought shape to use.
Only problem is it didn't come with any hardware. No biggie I can just buy some nuts and it will work. But it looks like it needs a gasket on the passenger side header flange. Can I just re-use the stock one or should I get a new one? If I need to get a new one where should I get it? Should I just get a stock one from Ford? Anything else that I need? I heard all 04s need O2 extensions with aftermarket mid pipes so I guess I need those too then?
How much louder will this make the car? Its the catted version. I don't mind it getting louder in the upper rpm range, but I'm concerned about idle and cold starts. My neighbors aren't too pleased with my magnapacks as it is.

Oh and this came off of a 99 Cobra. I checked around and it seems the Cobra uses the same midpipe as the GT. Will I have any problems using this on my car?

 

You can re-use the old gasket, but it’s not recommended. I’d head down to your local auto-parts store and pick on up (or, you can go to the dealer and pay 3X more). And yes, I do believe 04’s need an extender for your forward (or maybe it’s aft - can’t remember now) O2 sensors (might try the ole auto-parts store for this too, or any reputable online Mustang vendor).

As for sound, you should have very good quality and be very deep. I think you’ll be pleased.