Two mufflers and headers makes me think your a v6. Don't do any modifying to your automatic transmission, its sucks. They have a huge failure rate...
You'll hardly gain any horsepower with these mods. Intake headers and exhaust are only SUPPORT mods for turbo or N/A builds. Stockwise, it doesnt do much but make your car loud.
dont waste your time on mufflers unless your buying a header back kit. just sounding like your big doesn't mean you are. I think it's safe to say that there are probably quite a few people on here who bought a muffler package and now wish they hadn't changed out the stockies on the V6. I am one of them.
I/H/E will give you power on the dyno, a very small amount ... you don't really see much of a difference on the streets, that's your mind playing with you because now your car sounds louder
The ONLY time I felt a SLIGHT SLIGHT increase in power was with my velocty stack and K and N intake with filter. Thats it...all my other mods are probably just for sound.