1973 Dart Swinger Gas mileage

17 is pretty good
With 2.45s and optimized timing you might get 20 or perhaps more..
Just drive slower. Every 1 percent slower speed translates to roughly 1/2% better MPG.
Traveling at 60 instead of 66 is ~10% difference and is apt to increase your mpgs by 5%. Starting at 17 this means 17.85. The same can be said about taking out 10% gear and cruising back up at 66 . So, 2.94s might get you the same 17.85; while 2.76s might get you 19.9. If you combine both, then you can double up eg 10% slower and 10% less gear totals 20% rpm change and therefore 10% mpg improvement.
This only works at "normal cruising speeds" cuz the faster you go the more power it takes to overcome wind-resistance and you can't expect 19.9 at 100mph. But if your 3.23s got 12mpg at 100, then it would again be comparable, and the math says 13.8 with 2.76s.

As to the engine; with 3.23s your rpm might be 2700@65. So you need to optimize your timing at 2700, possibly at the expense of all other rpms. Then you just lean out the carb until it pukes. It is extremely important that your vacuum advance system is working. In fact, this is your primary weapon to getting the timing optimized.
If you have a heated air intake; in PEI, I would tend to keep it . It is designed to keep your inlet air temp to one temperature, namely 85*F, IIRC, so the factory can jet the carb lean and not melt the pistons in winter. Yeah you can make more power without it. And more economy with colder ,denser air, but then you better be careful between the seasons.
And finally, cruise-control sucks in the hills. It will be constantly sawing on the gas pedal. You would think it would average out ok, and maybe it would..... if the accelerator pump wasn't spewing every time, and the metering rods weren't jumping up.