1. fuel rails do not need to be modified.
2. there are two part numbers - the one you'll find on clubGP, and the one that I was given by SPO. i know which one I trust more
it seems that the number changed a little bit, but people ordering from gm parts direct will still prolly end up with the right part.
3. as stated above, lower intake manifold is common. you will need an adaptor for the throttle body.
4. the aluminum UIM is actually cheaper
than the composite, even if you buy a third party (ie dorman) intake. so the real trick is how much you spend on the TB adaptor. if it'* cheap enough, replacing with the aluminum upper will be more economical than replacing a failed composite.
5. if you don't have one already, you will need a cai when you're done. the neck of the aluminum upper is longer and has a slightly different angle to the throttle body. throw in an adaptor, and the stock airbox is pretty close to useless.