mpg single vs super six

Where did you research? slantsix.org has some very good articles on mpg.

comparing these 2 accurately will be difficult -- due to the age/condition factors of the items.

I do not believe there would be much difference between a 1 bbl v 2 bbl if all other factors are the same.
a 2 bbl would be more fun,
to get max mpg -- a lot of careful work must be done to maximize the mpg.
distributor (timing curve)
condition of the carb
exhaust
gears
manual vs auto
These are just a few factors involved & very general .
good luck in your quest , Lawrence