An adapter for a 3 bolt carb to a 2 bolt was a rather popular item back 50 something years ago. Warshawski Bros as well as other vendors sold them. A Ford/Holley with the load the load a matic disabled and replaced with a manifold vacuum port did wonders for a 235 6 cylinder. The same 3 bolt Holley came on a lot of 302 engines, but it is a too much carburetor for a Chev. A carb from a 54 239 Ford worked fine on Chevrolets as well as the smaller GMC engines. I have never heard of the barrel rule with cylinders. All I have ever seen on 302 engines was a 3 bolt Holley or a 4 bolt Stromberg.