[Retro Jim]

Carb jet swaps won't really make any difference to be noticeable in your fuel mileage . Like said before , upgrade the ignition system with a hotter coil . Electronic pointless ignition will also help . Better spark plug wire than the small stock ones will help as well .
Now one thing that will hurt you fuel mileage will be the 3 speed trans and rear end gear ratio you are running . Can you tell us what gear ratio you have ? Should be a metal tag on the top of the 3rd member under a bolt unless it was removed or broken off by someone which was common .
If you upgrade the trans with an overdrive , that will help the mileage . Now if you change the rear and use a gear ration in the 3.00 to 3.25 ratio like in a Ford 8" rear , that will help the mileage by keeping the RPM's down which in turn will help the fuel mileage .
If you do change the rear , get a Ford 8" rear from an early Mustang which are usually a 2.79 gear ratio and some were a 3.00 ratio . Cougars , Mavericks and a couple others will work as well but the Mustang & Maverick rears will work plus they are super easy to find . You can buy the complete rear for under $200 most anytime . The width is the same as your stock Dana rear you have now . There are no gears that you can swap for your Dana rear you have now . Plus , No , a Ford 8" 3rd member will not work in your dana 8" rear . Many have asked but won't even come close .
This are the best ways to get better fuel mileage from your Y block ! That way your engine will look pretty much stock to most people . If you don't worry about the stock look , then consider swapping a T5 in place of the 3 speed trans .

Jim