My old 75 highboy got 8 on average, it had a 390 4 barrel swapped in, but it sure had plenty of power and could pull a trailer without much effort. That was back in the days of cheap fuel though... That is the downside of FE's, they tend to be thirsty. I had 4.10 axles as well, may be something to consider, getting a little more highway friendly gearset if you aren't going to work it hard.