I've read that the reason for the upper intake failure on the 3800 is the coolant that goes through the plastic intake for the throttle body, so when I did my gaskets and manifold for the second time I figured I would block it off. 1/4 inch NPT plugs and NPT tap from harbor freight pn 91395.
I think it was the next drill size up from 7/16 in my drill set.
Iwanted to get the plugs i saw people use on the internet with the allen head but I wasn't able to get them locally so I ground half off the square part. There needed to be less than about 1/8th to fit in the upper intake. I put a bunch of black RTV on it.
Dorman pn 615-180 is the manifold, comes with the gaskets preinstalled, pcv and injector o-rings but not the upper ones so autozone had a oring multipack.
Aluminum intake elbows; another common problem the plastic original ones are like the intake they get hot and brittle and come apart. Dorman pn;47065hp or NAPA bk6601942.
1998 Bonneville SSE
2007 Monte Carlo LT-KYB, Clarion, Polk audio, Kicker
1999 Yukon SLE-KYB, Air lift, Reese dual-cam, Tekonsha
1987 Firebird-355/th400-nothing stock