Do lots of research before swapping a plastic L36 for the aluminum L26 upper intake.
This is not a straight forward swap and requires a throttle body adaptor and other considerations.

I have read about swappers going back to the L36 plastic because of problems.

Just replaced the upper intake for the second time on my 98 Bonneville. I didn't know any better and took it to the dealership the first time. I know for sure that this time the mechanic I went to replaced it with the Dorman 615-180 UIM. Does anyone know if this is going to be a problem another 30,000 miles down the road? Or does the Dorman UIM hold up pretty well? Thanks for any input.

There are a few aftermarket kits which work fine, the Dorman is one of them. They usually include some treatment for the EGR valve in the manifold. In fact all rebuilt 3800 II engines by Jasper have 2 things. One is the Dorman, and the other is they DUMP THE DEX for universal coolant.

IMHO, Everybody who buys a '95-99 SII N/A should plan on having to replace the plenum and the upper and lower gaskets. (The LIM gasket should be from the Dealer, they are all aluminum or have aluminum surrounds.) There are tech articles on it here.

If you are wrench challenged like I was 4 years ago, it'* better to ask for some supervision from a nearby member. If not possible, there is nothing fancy to it if you just take your time. With zero wrench time in 40 years (since i was 17), it took me 2 days, 4 hours a day. After doing it once, I bet i can cut the time in half. The veteran wrenchers can do it in less time than that.

It'* great experience for getting to know the car. And you eliminate the single tragic flaw of the 3800 II, on your way to a 200K+mile ride..