What MPG are you 340 guys getting?

I'm seeing whether my 340 duster would get some decent mpg since it's coming along together. Nothing crazy, other than a stock 68' 340 with edelbrock performer intake/600 carb, rebuilt 904 transmission. I honestly forgot the gearing in the rearend, but its in the vicinity of 3:4's. Since it is a 3 speed, think it's possible to get at least 18mpg on the highway?