I had a 2whl drive blazer, 350 + 373 gears. I built a motor for it stock heads, quadajet carb and a 260 comp cam. Milage was not as good as 16 but I think it was pretty close. It would tow my buddies ski boat great.
My buddie with the boat had an older blazer that we put together the same motor trany package except it had an aluminum intake,Edelbrock carb that was recommended for the motor/intake, the same 260 comp grind and 3.08 gears
He drove to Palm springs with the boat behind him and got close to 16.. but he was going 70... Tried to keep the motor around 2000RPM max
To get milage on the highway. Drive the speed limit and use the cruise. My 2007 F150 gets decent milage if I do that.