BS. If you want to put a 4 bbl on a /6, do it. The Mopar Six Cylinder book has recommendations for a 390 cfm Holley on an Offenhauser manifold for street use in the Slant Six section. The recommendations are for carb set up primarily. They also recommend the 72 and later stock cam.
The thing that the book does not address is the issue of kickdown linkages if you're using an automatic. Lokkar and Buchillon Performance make kits that address this issue.