Welcome to the crazyness! Car looks like a nice clean slate to start with. Shorty headers are a lot less restrictive than the stock ones, and you'll mostly notice more noise than power honestly. I swapped to Longtube headers, lost a little torque down low, but the top end greatly improved overall and the mighty 5.0 actually pulls hard in the higher rpms now. If you're not going to do any motor, ie heads, cam, intake, you wouldn't really benefit a great enough gain to justify the money for the headers and labor if you're not doing them yourself. If you're going to rip into the motor and build it up some, than 100% you should, get aftermarket shorty headers. Instead of spending money on the SLP catback, which is around 400-500 if I remember right. You can get a set of 18'' Long exhaust tips, have your factory tailpipes cut and those new tips welded on, and since you want SLP LM mufflers, have straight pipe welded onto your exhaust in place of your factory mufflers. SLP's are just open pipe, nothing inside. So maybe for new tips say $60, and add labor for that and straight pipe welded in place of the mufflers, you'll probably spend under $250 all together and get the sound you want. Just some food for thought