Between the Holley carb and big cam I think low 20s will be doing good. Holley carbs are bad on gas but yours is a little small so mabey it will be alright. The cam will make a nice compromise between HP and MPG. It has almost 30 degrees more duration than mine and that will knock off some of the super low rpm efficency.

I am guesing the tranny is a 833 od, that should help. .70 X 408 = 285 so not bad on the cruise prm VS displacement.

Are you running a 2.76 or similar gear?
Headers?