Not sure if I can be of any help, especially since this thread is a tad old, but I'll try since I hate to waste knowledge, especially since I recently I have learned a lot about the UIM & LIM for 96 and up 3800 Series II
I am not sure when, but I did hear GM came up with a first, and second design LIM Gasket, the second one is supposed to be the best from what I have learned, felpro makes a decent LIM gasket, but I prefer GM parts myself.
You can get a UIM that has a better design, the EGR stovepipe port has a steel bushing in it to protect it from heat a bit, it also comes with a reduced diameter stovepipe, a tip for putting it into the LIM, use your old stovepipe after you remove it of course, it should fit over it better then the deep-well socket method.
If you want to know if the car you plan to purchase has an aluminum gasket, just get a multimeter, and set it to Ohm, then touch only the gasket, you should be able to see a tiny bit of it, enough to test, it should show you 0 resistance, is so then you know the gasket is not plastic.
So far the best parts for this repair I've found are below, the LIM gasket is GM'* 2nd design.
If anybody ever buys one of these cars with this possible issue, and want to fix it themselves, the repair is actually not too hard if you have a few tools, I am by far a mechanic, but with the net & a good AllData manual, I was able to get all of my stuff removed, and I'm in the process of putting it all back together.