If you have a 99 and want the same LIM, you need the one with the smaller EGR bore. EGR bore 95-98 was .750" 99-05 was .625". The APN kit will come with a pipe that measures about .500" with a base that is about .749" That pipe is made to fit the older LIM directly. APN figures with a sleeved upper, that the stock .625" stovepipe in the 99 and newer LIMs provides a sufficient gap with the sleeve to make a good repair, and they are right. That is, after all, the same gap that GM provides from the factory with NO heat shield to protect the UIM. I like the idea of an even larger gap when I can get it, so, I recommend that owners of 99 and newer cars buying the APN kit, arrange to take the stovepipe to a machine shop and have the .749" base turned down to .624" so they can benefit from the same larger insulating gap (about .125") that the 95-98 guys get.
I wish I had both the newer and older LIMs to compare. I am pretty sure they are interchangeable, as I have spoken with GM service techs who have replaced the older LIMs on 95-98s with the newer ones just to get the smaller stovepipe. (Can you imagine that they would change out a $200 + part for a stovepipe!) So, I am pretty sure that the new LIM will fit on the older engines, and I would suppose that the older would fit on a new one, too. If it would, you could buy the older LIM and the APN pipe would drive right into the EGR bore. I would hate to advise this and have you find some difference in the older LIM that would prevent its use on the newer engines.
Can I ask, why are you replacing the LIM? Is your old one badly pitted where it mates with the cylinder heads?