Carter RBS is a good carb. From the Tech Archive:
First, lets go over your carb choices for a one barrel. In the early years Ford used Holley-1904-1908-1909 carburetors from '60-63, and the more common Autolite 1100 from '63-69. All of these carbs had 1-7/16 inch carb bores. The Autolite 1100's were offered on the 170 and 200ci, with three venturi sizes (1.00/1.10/1.20). Mustangs normally had the 1.10 or 1.20 depending on the model year, configuration, and engine size. In '69 most sixes got the larger Carter YF carbs, which were first use in California to lower emissions on the '67-'68 model years. However, some models came with the Autolite 1101. Ford switch to the more common Carter RBS for the '70-'73 models, which came on all 250ci engines. The 1101, YF, and RBS carburetors were larger, with 1-11/16 inch carb bores.
The Falcon Performance Handbook states the D5 head should have a 1.75" carb opening.