That sounds right on par with what you should get.
The 305 (5.0) and 350 (5.7) were gas hogs, but were good motors, especially the 350.
My 350 used to average about 12 not pulling anything. With my boat, I was 8-9 as well and that boat was probably in the neighborhood of 2500 pounds. You have the smaller V8 and 1000 more pounds. Plus, 65mph is going to make a huge difference. You're probably turning high 2000s, if not 3000 rpms to do that.