340+ for the STREET

Google Hot Rod Dave Super Duper mileage.He did an extensive sm blk. mopar build documenting results for each change.I think he ended up at 30 plus mpg!
No..an overdrive is not always the answer.I have a friend that installed an overdrive and his mileage went down.I think it was mentioned but your cam and gears play a big part in your mileage.Hi lift,short duration usually is best.Your cruise rpm must be at the rpm your max or most efficient lo speed torque is produced.
The easy way is to follow Rusty's post and use a modern grind stock style cam.