3.23 or 3.55 Behind a 318

I know guys that like the 408 in a truck but not so much in an A body with stock tires.
Few 408's get 20 plus mpg which is important to some guys.I just built a 372 and am
wondering how efficient it will be.We were very careful with parts selection.My stock 340
is over 20mpg driven sensibly.