First, you do need the rear o2 if you don't have a tune; it says '01+ cars for a reason. Pretty sure '04-'08 is still in that catagory.

Moving on, get some hotter range plugs for one, TR55's are too cold.

Second, The flex in the SD's are known to blow out, just go have the flex cut out and pipe welded in there, be a man...screw a flex.

Third, Flip the motor mounts in the dog bones...if you're mounts are older/tired/worn then there is prolly enough movement under load to cause things to scrape or make contact; example: downpipe on sway bar or o2 sensor on firewall. Easy fixes for that...1) Flip upper dog bones to lessen engine movement back and forth...2) get a C5 corvette o2 sensor and splice it in, as they are super short and small...3) remove the sway bar and flip it upside down to give a couple more inches of clearance.

Bam.