'66 273 BBD Carb

Yep, you screwed the pooch!

I believe the screw in the hole is a Allen screw.

Get that out then you MIGHT be able to with a left hand drill unscrew the mixture screw.

I had the same issue with a CAP 67 Carter 4 bbl. ( I bought it that way)

The other issue you might run into is finding a replacement screw

good luck