The 04 SS Spoiler will fit on your 03 ls because they are the same body. Thing is im pretty sure the SS spoiler requires 2 center bolts that the lc spoiler doesnt. The 2 bolts on both sides are there.

I am in the process of putting on a SS spoiler on my 05 Impala base sedan, so i've been trying to find some measurements about the angle in which the spoiler should be mounted. Do you know?

 

We just did one of those, it came w/ some plastic install guides you put in the spoiler and tape to the trunk... I doubt we still have them, but check a GM bodyshop to see if they sell just hte install kit for a spoiler.
I'll keep an eye uout for them later this week once i get the car out and start cleaning, but dont hold your breath, i think they got pitched..