I'll take a stab at it. It's been years since I've rebuilt a carb. and they were of the four barrel type. My 61 Apache's model "B" (manual choke) Rochester was leaking gas so I rebuilt it and everything works great now, amazingly. However during assembly when I got to the part of the flange gasket "between the carb and intake" the instructions say "chevrolet 6 cyl. with heat insulator block does not require a flange gasket. Mine had the block so I did not use the gasket. Are all of them "6 cyl" designed to have the block. It seems the additional heigth of the carb. would not only keep heat off it but possible allow for more of a venturi effect for the draw of fuel for idling.