NOS carburetors. What issues to be aware of if any?

According to the Carter BBD Service Manual it says the super6 and 318 BBD's are not interchangeable.

If your carb is configured for automatic choke, the choke lever is different/not compatible; you'll have to figure out a workaround for that. Depending on the year and spec of your V8 BBD you may or may not have difficulty attaining a good idle speed adjustment. If you can clear those two hurdles you'll be fine putting a 273 or 318 BBD on a Slant-6.