Howdy Cisco:
It's might hard to get improved mileage and performance from the same carb swap. FYI your stock Autolite 1100 is rated at 150 cfm and is married to your stock Load-O-Matic distributor. If you're read the links that Jack offered you'll get the picture. If you want a step up in carb capicity you could upgrade to an Autolite 1100 from a '65 200 with an automatic trans. It will be a direct swap and compatible with your stoci LoM distributor. The 200 carb is rated at 185 cfm. You can tune it for max efficiency, but will likely still be down in mileage- mostly because you'll enjoy it too much.
The other option is to upgrade to a Holley/Weber carb and a compatible distributor, like the DuraTech II or the DUI. If you have the earlier block you'll be limited to the DUI. Tuned right this combo should result in a giant step toward your goal. The holdback will be the two speed auto trans. It is not friendly to either performance or economy.
Keep us posted on your direction.
Adios, David