2.8 to 3.4 gas mileage

i'm swapping my 2.8 to a 3.4 in my 86 chief that has 4.11 gears. do i need to change to overdrive also or will the mileage be ok with the 3 speed 904. i'm building a daily driver and need as good mpg as possible.

should this be in oem tech

I ran a 3.1 stoker with 10.1 comp and a beefy cam in my trooper and seriously those engines wont have much tq to pull a big od so check s series forums and ask those 60 degree guys they know there stuff. pm me if you need anything else 60 degree related