If your looking for a little performance and mileage I'd go 273 if you just want mileage I'd go with170 and the a833 OD tranny and the highest rear gear you can get on either.
Weight and aerodynamics will be the ultimate factor
I don't care about performance I have other cars for that but I have a 318 2 barrel and 833 od tranny but I don't know what kind of gas mileage I could get