Would like to get my 66 Cuda's MPG high to drive Nor Ca to So cal

Taller gear probably best suggestion. A simpler way would be taller tires (and easier to switch back too).
Tall skinny 15s like the old Triumph TR6 used to run, something like that.
Going slower helps - at 60mph you get at least 10% better MPG vs 70mph. As engines get smaller this becomes true even more.
Best carb I know for MPG is the Holley/Weber 5200, was used on 1970s Pintos and other stuff. Progressive 2bbl, tiny little primary, adjustable air bleeds, I used to swap air bleeds up in the mountains so car ran great, all the way up Mt Evans, 14200 ft altitude. Lots of 6cyl folks use this carb, swaps on with an adapter.

Will the gas savings be worth it? Depends, 1200 mi round trip is 60 gals at 20mpg, you might save 10 gals or maybe $30/trip. Tires cost a fortune now that we have 2% inflation, about the only thing free is changing jets or timing.

I do find that car weight means almost nothing on the freeway (in the city it means a LOT) so I would not worry about putting your dog on a diet or leaving his favorite water bowl at home, etc.