So I have a Carter single barrel that gets awesome fuel economy and seems peppy

What type of fuel ecomy would I expect if switched to a two barrel carb??
I guess aound 8 to 15 hp if have upgraded exhaust just before drivers steering wheel to tail pipe.
Does anyone get decent or good fuel ecomy with these 2 barrel carbs used on a slant six??

If you are getting 8 you have other problems! What do you get with the single barrel. If tuned properly I suspect it will be very close the same.

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2 Mopars come with Spark plug tubes. One is a world class, racing machine. The other is a 426 CI. boat anchor!

12.70 @ 104.6

Sounds like a good recipe for "leave it alone, drive and enjoy".
This is incoherent. Fuel economy isn't measured in hp (which stands for horsepower). Are you asking about fuel economy, or how much horsepower you'll pick up by installing a 2bbl setup? Also, there's only one steering wheel, there's not a "driver's steering wheel" (and a "passenger's steering wheel"), and the exhaust doesn't go anywhere near the steering wheel. Please ask clearer questions.
Which 2-barrel carbs? There are many different ones in the world.
Yes, you can get good fuel economy with a thoughtfully chosen, properly configured 2bbl carb on a Slant-6.

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Too many people who were born on third base actually believe they've hit a triple.

Sorry I misread that? If you are asking about horsepower then it is about 10 horsepower. But 10 on top of only 90or 100 is definitely noticeable. With free flowing muffler and good timing curve and carb properly adjusted I would bet your mileage will be the same or better than stock.

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2 Mopars come with Spark plug tubes. One is a world class, racing machine. The other is a 426 CI. boat anchor!

12.70 @ 104.6

Without a good knowledge base of how to work on the car and adapt parts it is a bad idea. The posts from the thread starter/original poster over the past few months make me wonder as to the age of the poster.

This thread in addition to previous posts make me believe the poster to be a teenager with a newly acquired car and no knowledge on how to maintain/repair/upgrade it. If I am wrong I apologize but these threads give me this "belief." If my belief is actually correct then my advice is to leave it alone. Don't attempt to modify any parts on the car until the personal knowledge base is much more stable.

To answer the actual original question is nearly impossible. An early A body can vary greatly in weight which does affect MPG. Also there is the difference between 225 and 170 or manual vs automatic. In good condition I would reasonably expect 20+ mpg with proper tune and driver skill. This puts a large burden on driver skill. I routinely beat mpg ratings in every car I drive, I have a co worker that can't get within 10 mpg of any car he drives.