I get approx. 200 miles to a tank. The word Classic Mustang has no room for fuel efficient upgrades. I am not trying to be rude here. But if you are looking for efficiency with a small carbon foot print, you have the Wrong car.
rubbish. my mustang, after a rebuild and with the wrong cam choice, would turn in 18 city and 25 highway. with the right combination there is no reason that a V8 stang cant make 250-300hp and still turn in decent fuel economy.
the first thing you want to do is select a proper cam for your build. decide exactly what you want from your engine, for instance if you want a daily driver, then you want a cam that works best in the 1000-5000 rpm range. since you also want to improve fuel economy, you want a cam with more lift and a moderate amount of duration and valve overlap. something like a cam that has .460-.480 valve lift, and a duration of about 210 degrees at .050 lift. you also want a 110 degree lobe separation angle as well.
next you want a compression ratio between 9.5 and 10:1. you also want small port heads to keep airflow velocity up, something like the afr165 or windsor jr heads.
for an intake a standard edelbrock performer or weiand action plus intake will do just fine, you dont need anything more than that.
if you are going to use a carburetor, then stick with the edelbrock 500cfm carb or the autolite 4100(480 cfm 1.08 model).
run a small tube tri-y header, and 2 1/4" dual exhaust.
if you want to run something more than say a 3.00 or 3.25 rear gear, then you want an overdrive transmission. my mustang used the stock 2.80 gear to get the mileage numbers i got.
this combination, properly driven and tuned, should net you about 18-20 in the city, and 25-28 on the highway.