Whether or not it will get better mileage is anyone's guess. Any of the big blocks and small block won't be far apart.
I love my 455 Buick, just understand that performance parts are instantly more money when you talk Buick. A BBC or SBC is much cheaper to get parts for. In a Daily Driver I would probably get one of those since you won't have to order parts, with a Buick you will have to order most parts. You can't beat the torque of a Buick, though.
