why bother...get a honda if you are concerned about gas prices.
I'd say if you have a B or E body use the 400 if you have an A body use the 318. I would however not use 2.76 gears on either. lower gear doesn't always mean better mpg. Plus you'd have to drive it a TON on the highway to get any benefit. $5 a gallon is nothing compaired to the cost of building and maintaining a muscle car.