As far as I know, the 3800 Series I/II only used a plastic UIM. It wasn't until the Series III that the metal ones appeared, which to answer your question, I think was 2004. (You can put a S3 UIM on a S2, but it'* a decent amount of work.)
Not sure when the metal LIM gaskets started showing up, but retrofitting those is trivial.
what year did the redesign the plastic uim to use the smaller stove pipe so it didnt melt the uim ?? I know the new UIM I used on mine had the smaller stove pipe, I thought GM started uing that on the series 2 ??