I am at the stage in my PU restoration where it is time to decide what engine to put in. I have a 304 and 345 in about the same condition and I was wondering what I should put in. I want plenty of power but decent gas mileage. Did anyone swap one for the other and see a significant change in their mpg? Just for kicks the truck has a 3 on the tree and 4.09 diffs w/ 31 inch tires.