Carter 9626 mounting

You have the right idea with the bore size differences but when there very close in size, it is still a square bore. The minor difference is overlooked.

A true spreadbore is a TQ or Quadrajet. The dif is huge. Use a Edelbrock thick gasket. There a out a 1/4 inch thick. This will help keep the carb cool like the OE gasket did and seal it up well.