You might get 18-20 mpg without the overdrive. Limiting cruising speed would likely help with 3.23 gears. Personally, driving 65 is difficult because it just seems so slow!

I drove a mid to later 70's Chrysler Newport years back. It had a 400 with a 2 barrel carb. I'm uncertain on the axle ratio now, but that old car would cruise comfortably on the highway, so I'd guess it may have been a 2.76. That big old car managed to get a fairly consistent 17 mpg on the highway. I was pleasantly surprised.

The 400 is slightly bigger than your 383. The Newport may have been a bit heavier. Your 56 would likely have more aerodynamic drag. Your engine may have higher compression which would help fuel economy. Comparisons are difficult because there are many factors which affect fuel economy!

Looking at the distributor first is a good plan. Maximize your current combination and see what it will do before making major changes. The only down side to this is that changing the cam will mean redoing the distributor to obtain maximum performance and economy with the new cam.

An overdrive conversion is nice, but it's somewhat expensive on a big block. It would take quite a few miles to save enough gas to pay for the swap. Swapping in a 2.45 or maybe a 2.76 axle would be easier and cheaper. If the M-body 2.45 axles are to narrow or you can't find an 8 1/4 axle with this gear, some of the late B bodies had 2.45 which were wider. A LOT of the M-bodies with 2.45 gears had 7 1/4" axles. I believe the 8 1/4 was the smallest axle offered in the late b-bodies.