Daughter needs Gas mileage from 273 commando

I had a '68 340 dart with a .550 lift solid cam, x heads, large thermoquad carb (I built the engine from parts) 3:23:1 rear & I got 19mpg

the secret: 4 speed manual shift.

with a slop-a-matic trans forget it. 12mpg if you drive it easy, the camshaft will have very little effect I'm afraid.

you might be able to just do a torque converter change and get it up a little bit.
but before you invest tons of money into carbs/cams/marvel mystery oil/etc the best you could possibly do is swap to an OD manual trans. you will need pedal assy, bell housing, flywheel, pressure plate, etc. so it's not for the faint of heart