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

My 68 barracuda has a 440 and 727 from a 78 New Yorker. In the new yorker I could coax 11 - 13 mpg after deleting Lean Burn. I have not calculated the since the mild performance rebuild and putting it in my baracuda but I am pretty sure I can watch the gas gauge dropping...