Engine size recommendation

I'm looking for a little input on what direction I should go in for my 73 Duster. It is a 318 model with a non-original 318 and a 727 transmission in it now. It is tired and in need of a rebuild.

The intent of the vehicle is for me to drive it back and forth to work during the nice part of the year, during rain and snow it will hang out in the garage. It also will never be used to race anyone.

I want an engine that looks and sounds nice and is strong, yet gets decent gas mileage. I assume the 318 is the way to go, but if I use say a 360, will the engine get near the same gas mileage because it also won't have to work as hard and then will everything match up to the 360 (e.g. transmission). If anyone knows a good setup for a nice mix of power and fuel efficiency for this car please share.

Thanks in advance for the input.