1967 Barracuda - Slant Six (orig) or 440 Big Block? Daily Driver, Need MPG

MPG depends on your commute, city or highway. City depends on too many variables. You can get up to high 20's highway mpg with a /6, maybe 16 with any big block. Hard choice, I like my cars to move on their own, so I'd probably throw the slant six in and take my time with the 440. Everything is hard with the 440, driveshaft, exhaust, brakes, suspension. I'm thinking 9 inch front drums and a 7 1/4 rear with a 440.