400 HP out of your 3.8 would require more than forced air induction. Cam, valve train, fuel injectors, fuel pump, intake, exhaust, custom tune, just to name a few. And that doesn't get into everything you would have to beef up to handle that HP, like the transmission. My Trans Am with a 5.7 LSX engine runs over 575 HP, I have well over $15K into it (not including the cost of the car), and I still need a carbon fibre drive shaft and a stronger rear end to handle the HP. That engine started at 320 HP and is designed to be modded.

Hi 2kg4u, I agree entirely. Still don't want to make more than 205HP. Just want to start figuring out what kind of turbo to install and don't want it to work hard. My example of a turbocharger that CAN get me to 400HP but would theoretically be wastegated for 205 .

So, a Garratt TB0348 perhaps? It worked on the Gen1 3.8 . Can I set it to give me sea level? Maybe there'* a more contemporary option? Different size or something? Maybe it'* something I can pull from the local self-serve junkyard?

Yes pull a SC motor and trans then you should have 205 HP in the mountains.
and will only cost about $3000-$4000 with parts and labor.

OK so first step if your going to do it swap out a serious 3 intake manifold use a throttle body adapter they sell so you can still use your throttle body.
ZZ performance has them.
I would set a budget of $500.00 to do this you might get off at half that depending were you buy the manifold from.
Your plastic manifold cant handle any boost even 2 or 3 could cause coolant leaks into the intake ports from distortion.
Now if this is your only car you would be a fool to do this.
So that'* why I suggested a SC REGAL or Grand PRIX
So how many cars do you have more then this or???

Or you can do EL CHEEPOOO go to a pick and pull were they have 1 set price pull SC fuel lines, heads, valve covers, intake, balancer, pulleys, injectors.
For a top swap and get a custom tune for the ECU.
Then pull PS CV axel and diff and side cover out of the trans

Or you can try this
Leaf-Blower Supercharging a 1978 Chevrolet Monza Spyder! - Roadkill Episode 16

Nope I have others to drive while I fiddle with this.

So I think I see your point here about boost pressure. I hadn't thought of the significance of the fact that while at altitude while my engine is "at sea level" there would be boost pressure for the UIM to deal with. For what it'* worth it'* a fairly new intake with new gaskets etc. thanks to the notorious EGR issue. Does that change the equation any? Are there super duper UIM gaskets that I could change to?

Does a series 3 UIM bolt to a series 2 everything else (besides TB as you mentioned earlier)? Any modifications in the process? Same gaskets?

LOL not without swapping the UIM, right?

Someday I may get punchy and do something like this if a good donor car presents itself. Then it would probably be complete engine/transaxle/PCM out of a PA or somesuch. That will be after several other projects are complete and probably right before I shoehorn a Duramax into a Roadmaster Wagon.