Factory Carbie?

H'mmm. The Carter BBD used on all '72 318s isn't massive, and is more or less identical to the Carter BBDs used on your local Australian 273s, 318s, and 2bbl Slant- and Hemi-6s. Perhaps your car hasn't got an original carburetor after all. No, installing a 350 Holley is not "standard practice". It's one option -- probably not one I'd pick -- but it is not standard.