Recommended 4bbl carb for stock 318

You're getting answers from people who have done exactly what you are about to do, used the 1406 600 and had great success. THAT'S the WHY as to the answers you're getting. You couldn't be getting better answers. From actual experience. You cannot just "go grab a 600 carburetor" and slap it on. The 1406 is a very specific carburetor for specifically mild applications, like yours, and it metered to work on a pretty wide range of engine sizes, because of a well known scientific term called volumetric efficiency.

We're not telling you to go get a 600. We're telling you to get a 1406. BIG difference.