I was getting about 11 mpg on the highway with a similar 350, with a leaky old Holley carb. I recently switched to an Edelbrock 1406 carb, and now after the first couple of tanks I have gotten about 12 mpg in town (instead of the old 8 or 9) I haven't had a chance to take a long drive on the highway yet, but I'm hoping for 15.
At this rate, the Edelbrock will pay for itself in no time.
