'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