GM stovepipes from 95-98 are 3/4" stainless steel straight tubes that are tapped into the LIM EGR bore. GM stovepipes from 99 up are 5/8" aluminum straight tubes that are tapped into the LIM EGR bore. The reduced diameter stovepipe supplied by APN with their UIM kit has a base that measures .749" on its outside diameter, and as such, is a direct fit for 95-98 LIMs with a .750" EGR stovepipe bore. Your LIM stovepipe bore (99+) has an ID of .625".
The distance between the outside of the APN pipe, which measures about .500" or .505" and the inside of the heat shield in the EGR bore (about .750" ) is: (.750 - .500) / 2 = .125 ". So, using their pipe, you get a nice insulating gap of an eighth of an inch. If you use your stock 5/8" stovepipe, the insulating gap between the hot pipe and the heat shield is half as large: (.750 - .625) / 2 = .063".
Now, APN probably thinks that an .063" gap with a heat shield is good enough. Maybe so. But, if you want the extra protection for your 99+ L36, I advise that you take the pipe to a machine shop and have them turn the OD of the base from .749" down to .624". Then you can tap that little bugger right into your LIM. An important benefit of a larger gap between pipe and shield is the inevitable build up of petroleum coke that occurs over time as oil vapors are cooked on the hot surface of the pipe and build into the insulating gap between pipe and EGR bore. Where the gap is just .015" on the older cars, we often see the pipes carbon coated to the point where the gap no longer exists. Then instead of insulation, we have conduction. Not good.
Here'* what you will get if you have that pipe turned to fit your 99: (Thanks, Arbelac.)
If you use your stock 5/8" pipe, the gap will look more like this:
And, that'* if the pipe is centered in the UIM EGR bore. We have found several 99+ cars that exhibit an eccentricity between the pipe and UIM bore causing the pipe to sit closer toward the 2:00 O'Clock position of the EGR bore. We don't know why this is, but we would be interested to know whether you find the same problem when you install your APN pipe and UIM. If you do notice that your pipe and UIM are not centered, there is another reason to increase the size of the gap by using a smaller pipe.
Another option if the machine shop plan is not practical or too expensive. If you know or can find someone who bought a Dorman kit to repair their 95 - 98, they may still have the smaller, 5/8" base aluminum stovepipe that came with their kit. Maybe they would be willing to sell it to you for $10 or less? I don't recommend folks buy the Dorman kits because the EGR bore is not protected with a shield. But, because they are widely available with no waiting, they are by far the most popular choice.