Here's some pics of two 4bbl manifolds I built, one uses a spread bore adapter so Quadrajet can be used and the other just uses a square bore spacer, both manifolds were milled down and the bases were opened up. The Magnum manifold already has 4bbl plenum in it because GMC actually did offer a dealer installed 4bbl option so the 2bbl version of the manifold was just cast with a different top base
4bbl manifold for GMC V6.jpg
top view V6 manifold (2).jpg
0210001539.jpg
0210001514.jpg