When I used to work at the shop I replaced a upper and lower intake gasket on 3100, 3300, 3800s about once a week. My bonneville was leaking oil out of the sides (the valley between the block and lower intake) and I was getting coolant in my oil. After running some tests I confirmed I did not have a head gasket problem so I started tearing the intake off. Ive done LOTS of lower intakes on these engines but this was by far the worst Ive seen.
Here are some pictures - http://homepage.mac.com/spiralg5/
The only surface that was actually workable was between the lower intake and supercharger. Although wherever there was coolant ports there was corrosion. The surfaces between the lower intake and the heads were so bad I HAD to use a roloc to prep them, the scotch pad wouldnt even put a dent in the build up of crap. I estimate about 1/8 inch of crap was built up between the intake and heads. I also took the lower intake to the shop where I use to work to get it hot tanked so it would clean up nice, it did a pretty good job. Tomorrow Im going to do the same with the supercharger because the zzperformance coupler hasnt arrived yet, was suppose to be here friday =(
Im also going to paint the supercharger black just for the hell of it.
Just thought Id share my experiences, It took way longer than I wanted to spend but I rather get all the parts in good so leaks wont show up in the near future. Oh yes, and I used quality GM parts because I know from experience that the rubber strips from felpro have a problem of not making a good seal either.