‘66 Barracuda

Look at it this way: a 318 has 45 more cubic inches than a 273, and a 360 has 87 more cubic inches. Both can be dressed to look exactly like a stock 273 2-barrel. The extra cubes means more torque, i.e., more low and midrange, and that extra low and midrange is what you're gonna feel in moderate, part throttle driving. In short, the larger motor will feel a LOT peppier, even if the peak horsepower doesn't increase that much if you want it to look stock. And if you make just a few upgrades (four barrel, etc), the power increase will be huge. The 318 would be a more directly bolt-in swap, but the 360 has almost as many extra cubes over a 318 as a 318 has over a 273. Your choice.