Now, finally all the old stuff is out of the way and we're ready to install the new headers! This is a big step so pay attention.
Installing the drivers side header:
First things first. Using a 7/8" open ended wrench, remove the 02 sensors from your stock manifolds and screw them into the FLP's. Make sure you don't cross thread them, they shouldnt need TOO much force to be screwed in. Snug them down, and you're done.
Get under the car and remove the oil filter. No, your full engine's worth of oil isn't going to spill out, don't worry.
Find the steering shaft on the drivers side of the engine and you'll see a black plastic boot on it at the front end of it, where it meets the power steering rack. Either unclip it, or unscrew the clamp holding it in place and slide the boot up some so you have access to the 13mm bolt that goes into the joint. If your wheels are pointing straight ahead, the bolt will be accessable. Using a 13mm socket wrench, unbolt this bolt. Now, try to force the steering shaft towards the drivers side of the car as much as possible (it should move out of the way another 1/2" to 1" since the bolt is out and the joint can pivot a bit).
Next, remove the drivers side valve cover. On '99+ cars it is only held on by 4 8mm bolts. On LS1's (NOT LS6's) there will also be a PCV hose you'll need to use a flathead screwdriver to pry up and out of the grommet). If you have a 97-98, you'll have permiter style valve covers which have a few bolts around the outer edge of the valve cover. Remove the bolts and set the valve cover aside.
Now, after all this work, we can drop the header in from the top. It will take some time to work the angles out just right, but after a few minutes it should drop right in.
Look at the exhaust gasket you got with the FLP kit and orient it so the bolt holes are in the right place, and try to put the gasket in place and get a bolt started through the header and gasket. Once you get one bolt started, try to get another started. DO NOT TIGHTEN THESE YET. Once two are in, everything should be nicely lined up. Put a tiny bit of anti-sieze on the 4 remaining header bolts and hand start them too (DO NOT TIGHTEN!), then remove the first 2 bolts and put some antisieze on them as well and put them back in. Once all bolts are started, hand tighten them all first, then do a pass with a torque wrench at 18lbft (or, just make them hand tight if you think you can estimate torque :) )
Next, apply loctite on the steering shaft bolt and reinstall it. You may need to tap the shaft connector back onto the rack connector with a hammer if it was a tight fit to start with. Torque this to 25lbft.
Finally, fill your oil filter up with fresh oil and reinstall it.