I thought I would post some pictures of an Offenhauser 6228c intake manifold that were lost previously on this site. I had 2 of these but decided it was time to stop hoarding and sold them. They are much harder to find than a Kenne Bell or either of the Weiand's. The Offy 6228c is my favorite intake for a stockish Buick V6. This one had a casting date of 7/18/87. It has mounting holes for a spreadbore or Holley carb. Unlike the Kenne Bell and Weiand 7541 intakes, the 6228c only fits '79 up cylinder heads and it also does not have EGR passages like both of those have. The plenum is larger than a factory 4.1 intake but much smaller that any other aftermarket intake. The floor is raised in the center and contoured towards each port. The floor is also heated by coolant but can be blocked off. If you look inside of the thermostat opening you can see a second hole. If you don't want the intake floor heated, it was designed to simply put a plug with some sealant into that hole to block off the coolant.
I have a 1978 3.8. Seems that all 4 barrel aftermarket intakes are for 1979 & newer. I have installed a Weiand 7541, & it is running ridiculously hot. I have changed everything but the intake. Any suggestions or ideas?
You probably should have started your own thread on this which would have created more attention for you. It's going to be a long guessing game which also takes away from this one I started.
Because the radiator is cooling fine, it sure sounds like a water circulation problem inside the engine. Maybe it's a longshot, but being a new rebuild, I wonder if incorrect head gaskets or intake gaskets were used and the coolant passages are partiallly blocked off. Good luck.