Intake Manifold # question

You are correct. The D4b is for the 64/65 heads and the LD4B is for the later 273 heads. Although I have heard of others doing what a previous poster mentioned; hogging out the intake holes in the intake that you have a bit to make them work. D4Bs show up on ebay and moparts.com from time to time. My wife found a D4b for me for my birthday for around $75 on ebay. They are out there at reasonable prices. Way more reasonable than buying a new offenhauser that works marginally better than the factory intake.