I have a 66 so I’m all ears. My carbs have racing boosters with no tabs so I’m concerned about the mixture distribution too. I do have an extra stock front carb to copy the booster. Are you using the slot gasket front and back or 4 hole in front? Glad you showed that I wouldn’t have known. Must have been across the board carb because I have a 66 Cadillac AFB made the same way. It be interesting to hear why Carter engineers did this.